need some big block swap help

[fast_dev]

hey guys, swapping out the old 283/3 speed combo in my 70 to a 402 with a car 4 speed. keeping the stock bell housing so the problem im having is the mounts don't line up with the crossmember. i have the correct big block motor mounts so i know thats not the problem, do i just move the crossmember? i wouldn't think it would be riveted in different locations for different engine combos. also if i move it what size bolts do i use to replace the rivets? Thanks, Devin

[Tx Firefighter]

By far the easiest way is to get an automatic trans crossmember and mount it under the tailhousing of the 4 speed transmission. There will be two threaded holes underneath the back of the trans to do this with. The reason the holes are there is that the cars used a rear transmission and mount rather than the bellhousing mount like trucks did. You will have to remove the stock trans crossmember though, which can be a chore sometimes.

[fast_dev]

what would the stock setup be with a big block and a stick? same crossmember just in a different location? i got the stock crossmember out, but dont have one from an auto truck so im just going to try to use what i have. i know this may sound weird but im trying to build this from parts i have with as little cash put into it as possible, so far i got 12 bucks invested for a new throwout bearing, lol. Thanks, Devin

[70rs/ss]

With a stock big block placement is the mounts are in the forward holes on the frame rail and the tranny crossmember bolts where ever it lays (put it in and slide it under the tranny and use the holes that line up and drill the ones that do not. It is easy, and as mentioned about the auto crossmember is easier, but it can be done with the manual crossmember, just with more labor and gentle curse words and did I mention BFH (big f***ing hammer!) Three times the labor or a little more money, you decide.

[crazy longhorn]

The crossmember that you have wont work with the BB.....there were 3 different bell crossmembers(depnding on eng ), so you cant just slide them. The best route would be like TX firefighter said.....just chop out the bell crossmember, & use an auto crossmember(you can probably get one for around $25). best of luck.... L

[70rs/ss]

Here I go getting confused again. Does the 3-speed mount under the bell housing like the sm465? If it does, then you will need a new crossmember. The car four-speed you have mounts like an auto. I was thinking the 3-speed had a different mount than the 4-speed, and then therefor could be used. OOPS!

[Tx Firefighter]

70rs/ss, yes, the three speed mounts underneath the bellhousing to a riveted in crossmember like the 4 speed does. Funny thing is though, on factory 3 speed trucks, the transmission simply hangs off the bellhousing out into space without any support underneath the rear of it. Furthermore, the holes are there, in the trans for a normal tail mount just like all GM's typically use. The holes are simply unused in truck applications.

Someone more knowlegable than me stated on here the other day that there were 3, maybe 4 different manual trans crossmembers riveted to the frame rails of these old trucks. I knew of only two, inline 6 and V8, but there are different ones for the big block too and possibly the big GMC V6 trucks (that one's not sure).

I'd think a big block manual crossmember would be mighty rare to find though. That's why I was steering him toward an auto crossmember. That way, he just buys a 12 dollar trans mount and bolts the crossmember to his 4 speed, then drill holes in the frame rails to bolt the crossmember in as needed.

If he want's to do more work and save money though, he should be able to de-rivet the existing manual trans crossmember, bolt the engine to the proper engine mounts, and modify as needed his existing crossmemer till it lines up.

[hotrodmtodd]

I will tell you guys what I have come across in swapping my BB truck to an SM465 from an auto (originally 3 on tree). You will need a very RARE bracket for the block to mount the bellcrank in the factory position. The manual trans X-member would take EXTENSIVE modification to be used in the auto-trans/car 4-speed position because it is curved to mate to the frame in the far forward position for the granny tranny, where as the auto X-member is flat to match the flat part of the frame further back. If it wasn't so cold outside I would send you mine. I'm sure you can find one on here without much problem. If you try to use the "car hole" in the rear of a BB for the bellcrank, you will have to modify your floorboard hole for the pedal rod and move the frame bracket back. And you can almost FORGET about headers if you use the correct (rare) bracket. There is WAY more to it than you would think. I think it is the main reason why most guys go with an auto. A 3-speed bellcrank WILL work, but NOT a six. If you can weld, you will be able to make SOMETHING work. Good luck to you, Mike

[fast_dev]

update.... big block is in!!! you CAN slide the crossmember forward. heres what i did and what parts i used. big block towers, small block engine mounts, stock bell housing. moved crossmember up so the old front rivet holes are now the back holes. all holes lined up with NO drilling. ( 4 bolts per side, the other holes are already there) i did use the correct big block pivot ball and a v-8 bellcrank. (sbc and bbc are the same, i-6 is different). Hooker super comp headers. and a v-8 clutch rod from the pedal to the bellcrank. i did this same swap about 5 years ago and this is also how i did it then, i was actually asking to see if anyone did anything different to compare others ideas. i can provide pics if anyone is interested in the swap. p.s. last time i did this i didn't have the correct big block pivot ball, i took a v-8 pivot ball bracket and ground off the weld on the back of the ball and moved it to where it lined up with bellcrank and drilled a new hole to weld it into, i also still have that piece if anyone needs pics to do the mod. Thanks for all your input. Dev.

[stllookn]

Dev, Which car 4-speed did you use...M21 or M20? I had one of them in my 70 396 Camaro but never knew which it was...still don't. How do you tell the difference and which is better? Were there different gear ratio options for either of those transmissions?

[fast_dev]

i used the m-20 for this truck, better for a street driver because of the lower first gear ratio, easier to pull out. i have an m-21 in my race truck but that has 4:88's lol. both the 20 and 21 were the same strength, just different ratios. m-20 being a wide ratio and the 21 a close ratio. the m-21 will have one groove on the input shaft, all years, the early m-20's have no grooves but the late ones had 2 ( opposite of the famous m-22, early=2 grooves, late no grooves). i have the stock 3:73 rear in it now but im really thinking 3:08's for driveabililty, guess i'll have to see what its like first. Dev.

[71/454]

Sorry if this is a dumb question, but i'm doing the same thing but with a previously smallblock auto truck. What is the difference between a big and smallblock pivot ball? I assume that is just the ball that bolts to the engine? Also what do you mean when you say bellcrank? I have the conversion stuff from a smalblock granny 4 speed truck. Is that what others refer to as a z-bar? Sorry do not mean to hijack the thread, I just thought I had it figured.

[crazy longhorn]

Originally Posted by fast_dev

update.... big block is in!!! you CAN slide the crossmember forward. heres what i did and what parts i used. big block towers, small block engine mounts, stock bell housing. moved crossmember up so the old front rivet holes are now the back holes. all holes lined up with NO drilling. ( 4 bolts per side, the other holes are already there) i did use the correct big block pivot ball and a v-8 bellcrank. (sbc and bbc are the same, i-6 is different). Hooker super comp headers. and a v-8 clutch rod from the pedal to the bellcrank. i did this same swap about 5 years ago and this is also how i did it then, i was actually asking to see if anyone did anything different to compare others ideas. i can provide pics if anyone is interested in the swap. p.s. last time i did this i didn't have the correct big block pivot ball, i took a v-8 pivot ball bracket and ground off the weld on the back of the ball and moved it to where it lined up with bellcrank and drilled a new hole to weld it into, i also still have that piece if anyone needs pics to do the mod. Thanks for all your input. Dev.

The problem I see with sliding the crossmember, is that it changes the height of the rear of the trans(upsetting driveline angles). On the sb to BB , it may not be enough change to cause too much trouble.......but in the case of I6 to sb, its a 3 1/2" foward move that raises the crossmember quite a bit. yes you can slide the crossmember foward & bolt it in, but it changes the angles. there are 3 or 4 different bell crossmembers(factory) on these old trucks to compensate for which engine was installed. good luck, let us know how it goes L