Swapping a 350 turbo transmission to a 400

Generally no difference between the two. As long as it's a BPO trans (Buick, Pontiac, Oldsmobile) it will bolt up no prob. When I've done it in my Oldsmobiles all I had to do was grind and re-weld the mounting bracket on the crossmember. If they are both short tailshaft transmissions (they should be if the 400came from a car and not a truck), the driveshaft should be the correct length. The 400 will have a bigger yolk (the part that goes in the trans on the driveshaft) so you will have to use the one from the 400 on your old driveshaft. The local parts store should stock the u- joint you need to install it on the old shaft. The 400 also should have electronic kickdown (the 350 has a cable) and that can either hook up under the hood by the carb or just under the accelerator pedal. Have fun, keep in mind the 400, although a very tough trans, robs more torque from the engine.