SBC to BBC need help and/or advice please.

[JVictor75]

Finally decided to do the SBC to Big Block changeover.

I already know where the motor mount towers go, but I have a couple of other questions.

The truck (71 LWB C-10) has 1" drop coils in the front, and I need to know if I am going to need to replace the springs for going from mouse motor to rat.

How much further am I going to need to move the transmission crossmember? The truck has a TH-350. Also, will this transmssion hold up to a fairly stock 396? It (the motor) has a 3/4 cam, aka RV cam, and I'm going to put headers and a Performer intake on it.

Can I use the starter from my 350 on the big block? Reason I ask is that they are the same part number at Jeg's, Summit, and the others, and I'd kind of like to know. Also, will I have to "recurve" the HEI setup I have? And finally, will a 650 cfm Edelbrock performer carb work for now? Or should I rebuild the 750 Holley I have? Don't know the "model" number of the Holley.

While I'm doing this I'm going to go ahead and put the 68 front clip on, replace the heater core, the radiator support bushings and the control arm bushings. ( Control arm bushings are the only thing I didn't replace when I "fixed" the front end 7 months and 3K miles ago.

Anyone have any comments? Advice? Words of encouragement? Extra money floating around you just "have" to get rid of? Any of the above would be helpful and appreciated.

[Longhorn Man]

Did you get new towers too? I think the small block ones will mount it a bit too high.
Your TH350, if in good shape, will hold up fine. If it is NOT in good shape, you'll be on borrowed tiome there. I'm not sure if the flex plate on a 396 is double drilled, but if it isn't, you can slap a TH400 converter in there and it'll work fine.
While I am not positive, I am pretty sure your tranny won't move one bit, however, it will be up a little higher, and you might have to clear the floor boards a little. (BFH time)
As far as extra cash... I can't hel you. :d

[1971Stepside]

Not to hijack your thread, but the small block towers will work on the Big Block???? How far up does it make it sit??

[crazy longhorn]

The BB stands will move the eng back a bit from Sb position(they sit somewhere tween sb & I 6 position). the 350 trans wasnt an option behind the BB, so you may have to drill the holes for the trans crossmember, & also shorten the d shaft a bit. I wont say all, but most of the chevy flex plates had both patterns, & will fit all sb except the 400, & also fit 396,402 &427. The starter will bolt right on, & if its a high tork version it should work fine. im in the same boat on cash.......sorry, cant help good luck......crazyL

[crazy longhorn]

I dont think the sb stand /mounts will work with the BB......you could probably get it to bolt in, but I think you will have clearance issues around the ex manifolds or headers.......crazyL

[Longhorn Man]

Cool, thanks for the clearification Al.

[JVictor75]

Guys

I already have the bb mount towers. 68 - 71 style with bad bushings (rubber pieces). I also have the complete set of brackets and pulleys. Minus AC, but with power steering. How much longer is the big block going to be than the small block while in this position?

How much is a fuel pump, a water pump, and a set of (fairly) el cheapo exhaust manifolds/headers going to cost? How long can I expect the TH-350 to last? Do I need to go easy on it until I can get a replacement TH-400? When I do this, will I need to lengthen the pocking driveshaft again?

Do I need to replace the front coil springs due to the weight difference between the motors or not?

How much can I expect to pay to have the driveshaft shortened?

What, in your opinion, should I do to the truck or look at while I have the motor pulled?

[crazy longhorn]

JVictor75, the exact measure diff between BB & SB position , I dont know. From SB to I 6 position is right at 3 1/2",& the BB falls somewhere between those 2 positions. Your best bet would be to install the eng & trans, & slide the shaft in the trans till it bottoms,then pull it back 3/4-7/8".......then measure from the hanger bolt holes on the center bearing to the mounting holes on the crossmember to figure the diff in shaft lenght. If you have a 350t short tail(6" tail), it will be approx11/16" shorter than a short tail 400t......so if you swap to a 400t later, you will need to shorten the shaft a 2nd time. To shorten & balance 1 end of the shaft should run in the range of $75-125, assuming that there are no other problems. The BB should be in the range of 125-150 lbs heavier than a sb, so you might see a little diff in ride height, but I dont think it will be a problem. I would assemble the truck& see how it sits before changing springs. A stock fuel pump & water pump will probably run about $75, & my guess on the headers will be in the range of $125-150 for some cheap ones. For selection on the headers, you need to talk with the BB guys. Good luck......crazyL

[Desert Rat]

as you know you need to look correct
1.towers
2.ps pump and brackets
3.alt brackets
4.pulleys
5.fan shroud

you will need to move tranny crossmember back atleast in t400 applications about 1 1/2" if you have a 68 up frame they are already drilled for it.67 frames are not.also your trans linkage bracket on the frame is different but you can use the small block one,by moving it slightly thats just a matter of making sure your linkage shifts easily and tightening the 2 linkage bracket bolts.T350 as long as its in decent shape will be 100% fine.and if you decide to go T400 you will need another trans yoke,also theres no need to do anything to floor they fit in the small hump trucks.you can use the small block towers but then motor will sit funky and could run into angle problems and have vibrations from incorrect angle/pinion,my driveline i had shortened and balanced was $75 or $50 for unbalanced(i have had them not balanced and never noticed a problem,as long as reputable shop with a good welder and not big slag hunks on one side for uneven weight.BB manifolds i find at swapmeets all the time i pay from $1-$20 each depending on code and date people usually happy to get rid of them unless big block maniacs.but a set should be about $75 at average if you dont want to scrounge much.also the BB rubber cushions are quite spendy at about $150 for them with the rubber cushion underneath last time i bought a set.you can use small block ones and a big washer and shorter bolt(the washer to go over the bigger the hole in the bb towers) the sbc ones will save you over $125.Dont worry about your springs,when ive done swaps you really dont have much of a drop,its not going to make it a lowrider.Headers ive bought the $109 Flowtechs never had a problem,blown gasket or anything or burnt wires,but at exhaust shop i specifically request dont use the collector.and flange the pipes with the 3 hole bolt on deal and use a donut gasket instead of the crap flat gasket.water pump here about $35 for a standard pump,fuel pump regular stock flow under $20,650 carb is excellent for a fairly stock big block im running a 600 on a 402,it runs alot better than the 750 i put on there has more low end,criper throttle response,better mileage it doesnt run lean or anything.750 more when you get into more built or larger cubic inches. but if you have cam,headers,intake,good exhaust,a 750 wont hurt,Qjets flow about 750 IF you can get one that works right and has good throttle bushings and a top plate that isnt warped id go with one of those,but your 650 will be fine.Since your motors going to be out replace the front pump seal in the tranny(cheap i always do,but i like to replace rear main seal pan gasket,then you can look at the bottom end.throw in a new oil pump.If you have any BB swap questions shoot me a email im sure i can answer most of them,ive been there a few times. gmcrat427@yahoo.com

[GMC Jim]

Distributor #1112047 used with 350 Eng., Auto Trans has centrigugal advance curves 0 degrees @ 865 RPM up to 18 degrees @ 4200 RPM; vacuum advance-- 0 degrees @ 8" Hg., 15 degrees @ 15.5 Hg.
The 402 BB Eng (1971), Dist.# 1112064; Cent adavance from 0 degrees @ 930 RPM; up to 30 degrees @ 4400 RPM
Have the ignition adavance curves for various 396 engines....Need the vehicle description from which the 396 was removed and the distributor Part #..........can furnish the factory advance curves.............Jim

[JVictor75]

Guys,

I seriously appreciate all of the help. Can't answer the questions as to various part numbers I own, the truck is 250 miles away at the moment.

It will probably late Feb. to early March until I can get anything done with the truck but I thought I'd start buying little things here and there. You know, rad. support bushings, motor mounts, etc. If the stock fuel pump is going to be that cheap I will probably buy that too.

Thanks to everyone once again, I'm probably going to save the contents of your posts in MS Word to look at later.

One more silly question though. Don't laugh, but which water pump (long or short) am I supposed to use on a 396 with the stock 1971 pulleys and brackets? Stupid, I know, but I'm honestly not sure which I'm supposed to use. Once again, thanks guys

James Locke

[chevy2racr]

mine has a 454 in it now orginally 350. I didn't do i, but she has no fan shroud. has a TH350 behind it and I've pulled a flat bed with a 85 camaro multiple times. I've had a10Ft. slide incamper in it with a dek for a bd and she never seems to slip a beat. I dunno what all was changed but if you have points you'd like a messurementfrom drop me a message

[Longhorn Man]

As for tranny durability, it'll hold up, but the harder you push it, the worse off you'll end up. Lighting them up at every take off, second gear scratch... and real heavy loads willshorten the life, but we aren't talking about haveing to baby it all the time like it is gonna blow up when you floor it.
A tranny cooler wouldn't be a bad idea too...esp looking at your location.