Can I replace my TH350 tranny with a TH400?
Yes. The motor/tranny bolts should line up, but you do have to shorten the driveshaft. You also have to deal with the dreaded Throttle Valve Cable to control your shifts, instead of the vacuum modulator. You have options regarding the different mounting pad location at the rear of the tranny. If you can get a TH400 crossmember for your car, or there is a TH400 mounting position for your current crossmember, then you can get a replacement tailshaft extension from GM that puts the mounting pad where you want it. Otherwise, you can move the crossmember to line up with the 700R4’s pad and drill new frame holes to mount the crossmember. This way may sound easier, but some people have had problems with the height of the tailshaft, and had to fabricate or modify the crossmember. Also, the new holes in the frame have to be double-drilled. That is, you have to bore large holes in the bottom of the frame to get the bolts in, and smaller holes in the top, just like the factory holes.
