I assumed that my car has a t350 but someone said it should be a th400. How do I confirm which trans in in my car?

 

I dont know anything about older montes, but IDing a 350 vs 400 is easy.

Case lengths are pretty different although I don't recall the measurements.

Easiest way I can think is the trans pan. The 350 is a near perfect rectangle with one corner cut off at an angle. The 400 is an odd shape - maybe like the state of Indiana with a big bulge on the side? Definitely nowhere near a rectangle though.

 

Th350 was factory but easy way is to compare trans pan gaskets to figure it out.