Swapping TH350 for TH400 (Need Tips)

[1Bad_Monte]

Replacing my original TH350 with a TH400.  At the same time, changing to a tilt column.  Ordered a relay for the kickdown that mounts to the throttle cable bracket on the intake so that’s easy.  Anyone have any tips or pitfalls they can share?  
 

Still looking for a good TH400 and there are a lot out there.  Seems like if you get the trans brake included, they are reverse pattern.  Anyone do this with a horseshoe shifter?  
 

Posting pics of the freshly rebuilt TH350 that I cooked..

 

Thanks,

-Jr.

[72 Monte Carlo]

The T-400 is definitely a lot stronger transmission than the T-350, but outside of a shorter driveshaft and connecting the kickdown, it should be a simple swap.

[GRAY 70 Monte Carlo]

I have done this. There are some things you will need to get first as said in prior post is the drive shaft. You will also have to change the front emergency cable at the emergency brake pedal and the rear emergency brake cable. You will also need the large emergency brake cable C bracket at the transmission crossmember. All this is due to the th350 cables will end up right at your universal joint of the drive shaft the th400 moves it back further down the drive shaft.there is a th400 kick down harness that connects at the upper part of the firewall and goes to a kick down switch mounted on the acceleration pedal bracket.so you will need that bracket and that switch. You will need to check the speedometer gear in the transmission to make sure it matches you rear end gears to make sure your speedometer is accurate you can probably use the the gear from your th350 not sure if they are the same.good luck

[Scott S.]

Don't forget to change the input yoke.... The 400 and 350 have a different number of splines.  

[1Bad_Monte]

Awesome!  Thanks for all of the info.  The e brake is something I have been needing to address due to boxing in the frame blocks the cable.  On the list of items when upgrading to disk brakes.  The cross member is another good point and luckily that same frame kit added an admissible trans crossmember that fits most trans.  I found a relay that mounts to the throttle cable bracket so going to bypass the install at the pedal (appreciate the pics!).  Running a Dakota Digital dash and going to abandon the Speedo cable for GPS.   
 

Few.  Think I have most of the main things covered except the different shaft size.  
 

Now to find a good TH400.  Anyone had luck with the TCI Super Street Fighter?  Heard good reviews but also read a few questionable in the Summit Racing comments.  Next step up seems to be a FDI Trans and it’s hard to not want to get it with a trans brake.  Just don’t want to have reverse gears or have to use a race style shifter vs the horseshoe.  
 

 Scott, Greg and Doug, you guys are awesome.  I appreciate it.

[1Bad_Monte]

Update: A buddy of mine offered a TH400 that came off a 427 and said it ran strong when removed but had been sitting for a while in the barn.  Took it to the trans shop to get checked out and though intact, water got in it….  Looked up the numbers and the case is from a 69 Chevy Truck so the better alloy.  Debating a full rebuild vs FDI trans w/o torque converter (reverse gears with trans brake) is about the same price.

-Jr.

 

[Scott S.]

If you don't rebuild it, I'd definitely flush it and inspect it before installing it. 

Sitting water can get to places you can't imagine. 

[420ponies]

While your there on your frame and all the upgrades ,you should invest in this cross brace for the rear shocks.UMI Performance has it.

    

It will be a lot easier with the body off the frame for sure! And also add the Hellwig axle style rear sway bar (UMI Performance makes one also). Bolts to your rear axle with front heim joint connectors in front that bolt to the upper control arm crossmember. You'll be happy with the results (Ask me ,I know).

Edited August 10 by 420ponies
added content

[1Bad_Monte]

Awesome info!   Will take a look.  

[Scott S.]

The toes of knowledge...... 

[1Bad_Monte]

Well said.  The shop I had my engine built closed about a month after I received it.  Got little info besides the build sheet and a bill.  Going through all this taught me so much, not not only about my engine, but also gave me a working knowledge of automatic transmissions (more than I ever thought I would).  

-Jr.

[420ponies]

On 8/10/2025 at 2:41 PM, Scott S. said:

The toes of knowledge...... 

Well,at least there not white shoes like someone else.

[Dtret]

3 hours ago, 420ponies said:

Well,at least there not white shoes like someone else.

Or slippers. 

[Scott S.]

CROCS ?????