Are the TH 350 and TH 400 interchangeable with C3's?

 

No. Entirely different transmission designs. 400 is a MUCH stronger unit that can take a lot of torque/hp. 350 is a more 'basic' unit.

 

Thanks, but I mean does the TH400 physically fit where the TH350 was in these Vettes?

 

No. It is somewhat shorter and takes a different output yoke than the 400. You can research the differences on the web.

 

this is your 73 with many parts not there? it was originally a th400. if you have neither, don't waste time on either. 2004r thru 4l80. the electronic trannys need a controller, but they are cheaper and easier to find in the boneyard these days. buzzing down the highway at 3300 rpm is old school...

 

I just read that in 73 that if it was a bigblock car it was the th400 and if was the smallblock it was a th350.
Either way, agreed, I will be looking for a 4 speed auto.

 

i think they were all th 400 thru 76 or 77. the crossmember is removable?

 

The 700R4 (hydraulic O/D trans) is a direct replacement for the TH-350, I believe. The 200-4R O/D trans is a direct replacement for the TH-400. Either unit...or the electronic versions....will fit in your vehicle with some modest modifications. If you now have the TH-350 installed and do not want to go full electronic, find a decently built 700R4. There are some not-so-difficult changes that need to be made; but the driveshaft will bolt right up, I think.

 

700 needs 400 driveshaft shortened 3 or 3.5 inches. i think the 200 is a direct swap for the 350. and i think both are 27 spline. 400 is 31.but i am pretty sure all pagan has is a tranny mount. he is assembling his drivetrain from scratch.

 

yes, its a blank slate, and I will do what I want..just a rough old base model 73 Vette that I honor for its looks.

 

Not sure about the 200-4R but the 700R4 is not a direct replacement for the TH350. I swapped a 700R4 into my '79 many years ago and had to change just about everything. There is a factory bracket available (from an '82) that will bolt up the 700. ('82's came with 700R4 transmissions from the factory.) The driveshaft needs to be shortened and you will have some challenges with the exhaust system. The pipe will not clear the trans on the right side without a VERY tight bend in front of the crossmember. My solutions was to modify the crossmember to move the hole for the exhaust pipe outboard a couple of inches. It is also necessary to relocate the pulley for the parking brake cable that is welded to the crossmember. And of course it is a good idea to modify the shifter to allow for selection of all four gears, although it is not absolutely necessary. GM didn't do it when they put 700's in the car in '82.
There are also some differences in 700 transmissions. They came with different length tailshaft housings. It helps if you get one with the short housing like the ones that came in '82.
With all that said, swapping in a four speed automatic is highly recommended, assuming you prefer automatics. It completely transforms the car and makes it much more driveable.