TH350 to TH400 swap

[demian5]

Before you say "Put a 700R4 in it" please read the post...

My truck is a 1970 long bed with a small block and column shift. 307 rear diff with 31" rear tires. Currently building a 385HP motor.

I have a TH350 in my truck and I need to rebuild it since the motor will wipe it within minutes (its already slow to engage when cold).

I also have a complete TH400 sitting in my garage that I could get rebuilt as well. This would be a strong trans for my driver.

I rarely tow or haul with the truck, and the 350 would work fine rebuilt properly and upgraded where needed. The gearing requires me to put a stock stall converter in it, and I chose my camshaft for this combination (268H Comp Cam). Also, the 400 takes more power to operate than the 350.

If I decide to put the 400 in it, what do I need to buy and/or change on the truck itself to bolt the trans in?

1. Trans Kick down switch
2. Cross member - move back or need new?
3. Shorten driveshaft?
4. Shift linkage?
5. ?
6. ?

Thanks in advance - I want to do the right thing the first time.

[demian5]

I did a search before posting this and came up with no good info.

[tdangle]

Originally Posted by demian5

Before you say "Put a 700R4 in it" please read the post...

My truck is a 1970 long bed with a small block and column shift. 307 rear diff with 31" rear tires. Currently building a 385HP motor.

I have a TH350 in my truck and I need to rebuild it since the motor will wipe it within minutes (its already slow to engage when cold).

I also have a complete TH400 sitting in my garage that I could get rebuilt as well. This would be a strong trans for my driver.

I rarely tow or haul with the truck, and the 350 would work fine rebuilt properly and upgraded where needed. The gearing requires me to put a stock stall converter in it, and I chose my camshaft for this combination (268H Comp Cam). Also, the 400 takes more power to operate than the 350.

If I decide to put the 400 in it, what do I need to buy and/or change on the truck itself to bolt the trans in?

1. Trans Kick down switch
2. Cross member - move back or need new?
3. Shorten driveshaft?
4. Shift linkage?
5. ?
6. ?

Thanks in advance - I want to do the right thing the first time.

Should bolt right in.
You will get rid of the kickdown cable and install a trans kickdown switch and wiring
Cross member will move back. The holes should be existing
Shorten the drive shaft as the 400 is longer than the 350
I believe the shift linkage will bolt right up.

[kwmech]

Shift linkage and cooler lines will bolt up. x-member moves back about 2'', holes are already drilled. D-line will need to be shortened and rebuilt with a different u-joint at the trans. Kick down, not mandatory unless you're romping on it all the time.

[demian5]

Originally Posted by kwmech

Shift linkage and cooler lines will bolt up. x-member moves back about 2'', holes are already drilled. D-line will need to be shortened and rebuilt with a different u-joint at the trans. Kick down, not mandatory unless you're romping on it all the time.

Maybe I put a 400 in it so I can romp, on it and it lives longer.

[tj1970cst]

You could get that 350 built stout and not have to change anything.

[kwmech]

Originally Posted by demian5

Maybe I put a 400 in it so I can romp, on it and it lives longer.

Then you upgrade the pedal assy and install a switch

[special-K]

I'm a big fan of the TH350. If you want a TH400 I understand. Just do it. It's not like you can go wrong. But there is that lost power, which I see as a lot when you look at what you do to get that much more. And the TH350 has time-proven itself to be no wimp. It is basically the transmission that made automatics the standard they are today. The one that killed the manual. Add all the things that can be done to them for those who like to play and for me you just can't beat them.