Your Uncle is misinformed. Your car is equipped with the 8.5" 10-bolt and will hold up fine behind a 455. Rear tires on the other hand won't (LOL).
To swap a th400 into your car will require a little work. The driveshaft will need to be shortened and rebalanced, and the tranny mount will need to be moved toward the rear of the car. There are 2 ways to do this. Either move the entire tranny crossmember about 4"-6" to the rear (requiring you to drill new mounting holes in the subframe) or cut the mount pad off of the crossmember and reweld it to the rear side of the crossmember. A properly prepped th350 will hold up just as well behind a mild 455, however.
