TH350 to TH400 Conversion, Swap

[dag19]

Hey guys!
I am looking to put in a TH400 into my 70 C10 and would like some good insight from you all. 1. Will I need to put in a new crossmember? 2. Is the tail dependent on driveline length? Short versus long on tranny. 3. What years are the good years to look for? 4. And does anyone have a TH400 in the Dallas/Fort Worth area?!

[ChevLoRay]

I'm gonna say to the forum! I'm running a TH400 in my '69. They were introduced by GM in '65 model cars, and were installed until they were replaced by the 700R4's, which were superceded by the 4L60E. So, there's a lot of TH400's out there.

One question: Why a TH400? I also had another '69 that had a TH350, behind a 350 that was built and would chirp the tires on the 1-2 shift, every time. The 350 has a little shorter first gear and lots of people prefer it to the 400.

So, what are your plans and how does the 400 figure to be the transmission of your choosing?

[dag19]

Thanks a lot for the welcome note!! Well great news that there are a lot of them out there...I should be in luck! :-) Currently I run a TH350 but I have rebuilt it twice already and am ready for the 3rd time any minute now. When I am on the gas I can chirp both shift points but once I start to shift down manually it always stays in the higher gear and just coasts. Seems like it is slipping. It feels pretty sloppy on the way down and it shouldn't be like that. I haven't been able to figure it out.

So I guess since I have had bad luck with my 350 I am ready to go onto something more heavy duty and something that can stand the abuse. Thoughts on that?

[ChevLoRay]

There are companies that sell hi-performance rebuild kits for transmissions, and that includes the 350. Check out Summit Racings' website and see what you can find.

[dag19]

So you think my best bet is just to rebuild this one AGAIN but with a better kit? Any change to the valve body? Shift kit?

Do you have any ideas as to my described symptoms? Just bad clutches in there?

[Notrodder]

The 400's a pig. If you 're going to go through the hassle of changing, why not go with something with an overdrive?

[71 c10 rat]

Welcome to the forum. I'm running a 454 with a TH400, i had to get the drive shaft cut down about 3in from stock The TH400 is longer.(cost was around $218 for new U-joints carrier bearing and balance at a local shop here in WA). Also i went on Classicperform.com and got the tubular crossmember they sell for around $80 you dont have to have it but it made the job easier. As far as crossmember location c10's already have holes to move the cross member back to hold the TH400. hope this helps.

[dag19]

The main reason I haven't went with an overdrive is the fact that I rarely drive it down the highway or interstate. I don't take long trips in my pickup or anything so I thought it would be a bit of a waste to spend the extra coin to get an overdrive. But on the other hand I have always been out in the country where I could go 30mph and not have anyone honk at me cause I was goin too slow!

Will a 700R4 hold up under all conditions?

Originally Posted by Notrodder

The 400's a pig. If you 're going to go through the hassle of changing, why not go with something with an overdrive?

[dag19]

You bet that does help out quite a bit. Yeah I kinda figured I would get into spending quite a bit more money if I were to switch tranny's. I don't necessarily want to get into that. I am new in this area and will proabably have a hard time finding a good, trustworthy shop.

Spending that kind of money I could easily just rebuild my current tranny or buy an aftermarket tranny such as TCI or something a long those lines. So thanks for the good info!

Originally Posted by 71 c10 rat

Welcome to the forum. I'm running a 454 with a TH400, i had to get the drive shaft cut down about 3in from stock The TH400 is longer.(cost was around $218 for new U-joints carrier bearing and balance at a local shop here in WA). Also i went on Classicperform.com and got the tubular crossmember they sell for around $80 you dont have to have it but it made the job easier. As far as crossmember location c10's already have holes to move the cross member back to hold the TH400. hope this helps.

[Fitz]

Originally Posted by dag19

Hey guys!
I am looking to put in a TH400 into my 70 C10 and would like some good insight from you all. 1. Will I need to put in a new crossmember? 2. Is the tail dependent on driveline length? Short versus long on tranny. 3. What years are the good years to look for? 4. And does anyone have a TH400 in the Dallas/Fort Worth area?!

You may have to move your crossmember back just a bit but you won't need to replace it...My 67 frame was already drilled for the new crossmember location and I believe the '70 had multiple sets of holes for the crossmember too so you probably won't have to drill the frame. Whether a turbo 400 is a good choice or not depends on how you drive your truck and what engine you're running. When I swapped my turbo 350 out for a Turbo 400 it cost $60 at a local machine shop to shorten the driveshaft and rebalance it.

[dag19]

If I remember correctly I do have the extra holes in the frame for the crossmember. It has been a couple years since I have had to be under there!! I drive it pretty easy when I am goin down the road. When there comes an occasional race I hit it pretty hard but thats about it. Its always just few and far between when I actually drive it. I will have to check into a few shops to get a good feel on what they charge from shortening and balancing drivelines. Good info!