You did not go into detail about what the "innards" are in the rear end...a bone stock TH400 is good to about 450-475 ft lbs of torque, give or take....but a well prepped TH400 can handle over 1,000 hp. The drive shaft is usable, but I would change to a grease-less (sometimes called sealed)U-joint set up that are available through most NAPA and Car-Quest stores...most U-joints break at the internal grease passage that the Zerk fitting screws into. I have pushed a mild prepped TH350 and a 10 bolt with Richmond Gears and Moser Axles well past the 500 hp, 500 ft lbs of torque level and had them live for 100 passes down the track. It all boils down to how you are going to drive it, and what type of tire you have on it. A race slick will break parts quicker than a street tire that spins a bit first.