Auto Trans swap in 72 GMC - Need Help

[scorpion1110]

Gentlemen, I need your sage advice. I am getting my mind poisoned by the local truck guys. I am here in search of one or two gurus who can keep me on the straight and narrow.

So three weeks ago I buy a 1972 GMC Sierra Grande. My wife doesnt say a word because she knows that I am dealing with some serious health issues (the kind that involve biopsys and make me ensure my beneficiaries are current on all accounts). She thought the GMC would be good medicine and I love it.

Its a 3500 Camper special, 8 1/2 foot bed, factory tach, blue houndstooth, auxilary gas tank and electric fuel pump, wood bed and 4:11 posi. A nice solid old GMC. AND its a 4speed. Well I generally like rowing gears, but I find the 4 speed kind of annoying. I start it in second, but try and downshift well arggh. So i decide that the truck might just be a better driver as an automatic. I will run 31" tires and hopefully just driving local, revs wont be an issue and if it does go on the high way I will just deal with it.

The Plan is a conversion with a TH350. I know I need flexplate, shift linkage, column with shifter, trans, convertor and a drive shaft that accomodates the auto and an auto crossmember.

So I call a trans guy who tells me I need a TH400 and that I will blow up a Th 350, then a friend says a 700r4. Another says that a Muncie might be a fun swap (and I can build one thats tough) but I am not sure anymore how to go.

Now all I want is a TH350 and a truck that is a bitt easier on me to drive.

So Gurus, HELP!

1) can I run a TH 350?
2) if I built a Muncie M20, with bigger bearings and a cast iron midplate would it stand up to the truck.
3) if the Muncie and the TH350 are deal breakers than must it be a Th 400 or should it be a 700R4.

And finally I am talking to some folks in MD who have some of the parts, but if there was a guy with a rolling frame in the area that was set up as an auto, I would show up this weekend with Cash and take it (Hint Hint).

Thanks guys (and gals) for reading- I appreciate your help

S

[anviltester]

700r4 all the way. the overdrive will work good with the tire size and the gear ratio. if its a stock or mildly modded motor you won't have any problems.

[hugger6933]

Well first off Sorry to hear your dealing with what you are and welcome to the site. The reason most guys are gonna say it has to have a TH400 is because it is in such a heavy purposed truck. that T400 was the heavy trans staple for the years 1968-like 1992-3ish when the first of the 4l80 e were unleashed upon us.[they to oversimplfy are a 400 with an overdrive]. I am not aware of the th 350 ever being put in a truck that heavy but it certainly don't mean that you can't do it or have it done. You not gonna be pulling or towing large loads are you? The main thing for this truck now is driving? then most def the th350 is right on with what will be fine. Also your gonna need some sort of tranny oil cooler and cooler lines. Some guys are telling you the 700R4 because of the overdrive and the low rear gear that you have but that is not that big of an issue, and unless it is super built it is not the grand way to go in a heavy truck but we have already decided that it just is a driver so that may not matter either. The 350 is still the best looking plan in my mind and that is what you wanted all along. Jim

[cherokeejohn]

I don't have a rolling chassis, but I do have a running 72 C/10 350/th350 longbed 8". There's lots of details to tell. I dont know how interested you'd be in a swap, "possibly". I am located in Finksburg, about 30 min away from Mt Airy. I used to live on Watersville rd. Anyway if your interested, intrigued, or just wanna chat, take a look and take it from there. It's no cherry, but it is a good, restoreable, or drivable truck. I personally wish I had a 3/4 ton or larger as I need a TRUCK, I also love the longhorn/Sierra Grande. Anyways PM me if you wanna chat. John

[cherokeejohn]

Also... If installing an auto is the way to go, I do know of a really good trans shop that does custom swaps also, warrantied work, in my area, just into balt. county, in historic Reisterstown. "Whitcraft Transmissions" I believe Its called

[Z10]

I just made the swap from a T350 to a 700r4, which you can see in my build thread.

I had to shorten my drive shaft about 3", move the crossmember back, run trans lines and a trans cooler and use a flexplate. You'll have a very nice 1st gear and more importantly to me an overdrive.

The cost of my trans which is GM rebuilt from a 91 Corvette was about $1500.

Hope that helps.

[scorpion1110]

I did a little extra reading and it seems the 700R4 has electronics that need to be addressed- I want to try and keep it simple. While I wont be pulling trees with the GMC, I do want it to keep some of its character as a work truck. It sounds like I am going to have to go toward a TH 400 - is there anything weird about that conversion? I want to keep it simple as possible so is there a date range 400 and anything special I need to consider?

Thanks guys, I really appreciate your time and insight.

[MidLifer]

The 700r4 electronics are not such a big deal - TCI sells the kits, and a good trans builder can set it up so that all you need to do is mod the brake light switch to disengage the tc lockup when you step on the brakes. Alternatively you can put a toggle switch on the dash and manually engage and disengage lockup.

The other trick with the 700r4 is the TV cable, which is NOT a kickdown cable and must be properly adjusted or you will meet with rapid disaster. Again, not a hard thing to do, but you will need the right bracket for your carb. They are widely available online. Bowtie Overdrives has great info on their site about this. Check out their TV Made EZ kits.

If you want to drive at modern highway speeds, the OD is a big plus and your gas mileage will improve significantly. If you are just doing around town driving, the TH350 should be fine unless you are consistently hauling a ton in the bed or towing something huge.

[Eddie H.]

Another option to consider would be to try and find another truck of the same general vintage that came factory with a th400 to use as an organ donor. That way you get the trans, drive shaft, cross member, steering column, shift linkage, brake pedal assy, radiator, cooler lines, and everything else and just swap it over. You could then put all the standard trans parts back in the donor truck, sell it, and maybe break even, costing only your labor.

[JimKshortstep4x4]

Unless you plan on abusing your truck going with a TH 350 as you are considering will work very well. The TH 350 came in 3/4 ton trucks from the factory and if you go with a TH 350 you can use your existing driveshaft.

Jim

[scorpion1110]

Jim-

Th350 is what I originally thought and how I want to go though I have had a history of blowing up automatics

So the 350 like I originally had hoped will motivate the 1 ton? If I haul it would be plywood, drywall, fence posts stuff like that.

Eddie, donor trucks are hard out here. Chasing parts is a full time job

[Eddie H.]

Originally Posted by scorpion1110

Jim-

Eddie, donor trucks are hard out here. Chasing parts is a full time job

I guess I just assume that these old trucks are still plentiful everywhere, like they are here in Texas. I can drive a few miles in any direction and usually see several of them on any given day.

[Rude Ron]

Originally Posted by scorpion1110

I did a little extra reading and it seems the 700R4 has electronics that need to be addressed- I want to try and keep it simple. While I wont be pulling trees with the GMC, I do want it to keep some of its character as a work truck. It sounds like I am going to have to go toward a TH 400 - is there anything weird about that conversion? I want to keep it simple as possible so is there a date range 400 and anything special I need to consider?

Thanks guys, I really appreciate your time and insight.

th350 are really good tranny's and would be fine for your needs, but with that said you are better going with a short tail shaft th400 for all the obvious reasons and it sounds like it is a nice original truck and needs to stay that way, thus th400 no brain needed super strong etc. and if need be swap to automatic ring and pinion ration in the rear end down the road, Hope the health thing turns for the best soon no guru here but this makes sense

[SS Tim]

Thinking a P/M to Special-K will get you on track if not the parts you need. He is kind of close too.

[scorpion1110]

Guys Thanks for the well wishes- Big C kinda sucks, but cant dwell on it.

I spoke to two transmission shops and they didnt like the idea of a TH350 in a one ton. I know one guy personally so I am confident the info was genuine and not deigned to sell me a TH 400.

So hopefully my last question.

I found a Th400 short tail fresh from a builder who will warranty it. Great, but......

Its a bolt on Yoke. I have a two piece driveshaft with a yoke in the center at the support, but everything I read suggests a bolt on yoke is NOT THE WAY TO GO for a retrofit.

Am I OK with the bolt on if I keep the two piece shaft or should I opt for a slip on yoke type. I think he has both but am supposed to meet him to halfway to pick up so want to make sure I get the right trans.

Any suggestions on this? I am supposed to meet him this afternoon.

Thank you for the education fellas and the help!

S