4 speed trans?

[71c20junkie]

Hey guys this so my first post on here but I've got a 71 c20 350 with a sigsaw 3 speed and a Eaton ho52 rear i want to convert it to a 468 big block with a toploader 4 speed out of a 1 ton truck will bolt up to the he'll housing but what about a drive shaft of any other issues I might have I have the motor and trans already thanks

['68OrangeSunshine]

If you're changing to the SM465 Muncie ["GrannyGear"] 4-on-the-floor it should be an easy swap. Bellhousing is different.
SM465 has a 5-1/8" opening in the bellhousing [GM5]3925505.
SM420 and 3-speed Saginaws used a bellhousing w/ a 4-9/16'' opening [GM11]3815894.
I don't know if the propshaft will need to be shortened.
You will also want an 11" or 12" flywheel.
And you gotta have a High Hump center cab floor.

[71c20junkie]

I have the flywheel that came off of the 468 and yes it's a sm465 they came out of a 78 dually 1 ton I just don't have the bellhousing so I'll have to expand the hole I know I'm going to have to raise the floor up a little bit too

['68OrangeSunshine]

Originally Posted by 71c20junkie

I have the flywheel that came off of the 468 and yes it's a sm465 they came out of a 78 dually 1 ton I just don't have the bellhousing so I'll have to expand the hole I know I'm going to have to raise the floor up a little bit too

Yeah, a machinist once offered to open up the hole on one [a junkyard freebie] for a very nominal fee. So it's do-able. I had it painted up with coats of Chevy Engine Orange, not knowing there wasn't just one size. But I had another one w/ the 5-1/8" opening, stock, so I just painted that one and it's on my '71 GMC now.
The bigger bellhousing should be available in a junkyard, if the 468's PO is keeping his. They only made a zillion of 'em. Might scratch around for High Humps while you're there.
OK. So you may have to have the propshaft shortened. Take it to a driveline specialist and also have it balanced.
Does the Big Block have headers or stock exhaust manifolds?

[71c20junkie]

It's torn apart at the moment but I'm going to put longutbes on it if I can

['68OrangeSunshine]

Originally Posted by 71c20junkie

It's torn apart at the moment but I'm going to put longutbes on it if I can

Longtubes should work real good. C/20 3/4 Ton frame will give you space you'd kill for if you had a K/10. Look out for your z-bar action. Don't know if the SBC/BBCs have the same z-bars, but the L6 is a different animal. Hydraulic clutch would be your next option, but ''Ize don' know nuffin 'bout no hydraulic clutches.'' Way above my paygrade.

[71c20junkie]

That's what I was wondering but I think the z bar will fit just have to figure something out about a drive shaft

['68OrangeSunshine]

Check your local phone books for driveline shops and call around.
You will probably have to get your powerplant/bellhousing/tranny mocked up to figure out your propshaft length. Then have the pros cut and balance it. Good time for new U-joints all around.
Is it a one-piece or 2-piece?

[71c20junkie]

2 peice shaft did the sm465 come in the 67-72 trucks

['68OrangeSunshine]

Originally Posted by 71c20junkie

2 peice shaft did the sm465 come in the 67-72 trucks

Since model year 1968. '67s had the SM420. My '68 C/10 Stepside came ordered with a 465 and a 292 L6 and my '71 GMC Jimmy was produced with V8 350/SM465/NP205/12-Bolt Corp w/3.73 Eaton Posi.
Only notable difference is that the early 465 shift knobs read 1-2-3-4, later models' knobs read, L-1-2-3 [or was it L-2-3-4 ?]. Identical trannys.
First gear has no synchro, so it's hard to downshift w/o crashing gears. Usually drivers will only select First [or Low] if starting out with a bed full of bricks and faced with a steep incline.
Have you selected a clutch yet?

[71c20junkie]

Not yet I've got the stock clutch that came with it

[Mike C]

This is NOT as easy as suggested. Big block mounts are different obviously for the motor, and the transmission crossmember for the bellhousing is obviously different since the motor has moved and the push rod and bell crank are different for the big block as well. At minimum the front half of the driveshaft will need to be replaced but chances are good that a big block truck has a different carrier bearing so the whole shaft may need replacement.

Donor big block 4 speed truck is the simplest way to pull this off, but not easy to find. I would be looking for threads where owner with BB/4 speed truck is looking to convert to auto and see if you can buy all the parts lock stock and barrel.

[71c20junkie]

Hopefully I can change the front section of the shaft and leave the rear but I've got the stock mounts and big block mounts then I have to find the large hump cover so I can put that in as well the driveshaft is my main concern

[Mike C]

The manual trans big block piece is the hardest to find along with the big block cross shaft and the push rod from the pedal, but it is out there and a lot of SM465 guys want to go auto.

[71c20junkie]

Shouldn't the z bar and all the pushrods and stuff be the same and what big block piece are you talking about

[71c20junkie]

What about a Saginaw 4 speed or another 4 speed that's an easy swap

[Mike C]

Yes they should be all big block parts. But big block manual trans trucks are not near as common as small block trucks. And all of those pieces are different. A Saginaw swap is much easier if you use a rear transmission mount ala TH350. If you keep the small block and its mounts you should be able to bolt in the Saginaw using all the same parts including bellhousing and driveshaft I'm pretty sure. But if you go to the big block, you have the same problems because it moves the motor forward requiring all the previously stated parts.
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[71c20junkie]

So what if I just put the big block behind the 3speed and get big block transmounts and motor mounts

[Mike C]

That works, but you still need to find someone to supply the (reasonably) hard to find big block bellhousing/trans mount.

If I was doing a big block/manual trans (and I have thought about this for my 69 short step) I would get the big block mounts and a TH350 crossmember. I would mount the engine using the factory reproduction big block towers and use a TH350 crossmember. I would get a big block push rod from the pedal and a big block cross shaft. For a trans I would run a Super T10 or a new McLeod street 5 speed.

The crossmember mount holes will need to be drilled in the frame and a custom driveshaft made. On a stock height truck just a new front to the 2 piece would probably OK, but on a lowered truck I would go one piece.
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[71c20junkie]

But as far as me sticking the big block behind the 3 speed I just needth e mounts I have the big block towers I just need the big block trans mounts the rubber mounts or the cross member

[71c20junkie]

alright so ive decided to ditch the 465 and look for a siginaw 4 speed instead everything will bolt up nicely i know the big block towers just raise the motor they dont move the motor forwards or backwards so it should be fairly easy plus getting the big block front and rear mounts anything im missing i guess i wont know until i find a trans and bolt it all together