The powerglide has two different ratios that are stock. One is the 1.82, which is the light weight version (I broke a set on a 2800lb car running 10.00's). It has a stamped steel planetary housing and the input shaft measures 12 5/8" long. The second version is stronger and it is the 1.76, which has a thicker cast planetary shell and the input shaft is 12 7/8" long. I have run the 1.76 version in a 2650lb car that ran 8.90's. What finally broke it was when the rear gear broke and locked up the rear end, the output shaft of the planetaries was twisted off, but the gears were still fine. I now have the 1.80 straight cut gears and they cost about a 1000.00 several years ago.
What ratio was your gear set? Usually the one that breaks at that hp level is the 1.82, because the 1.76's can handle a good amount of hp (not the 1200 but usually 800 or so).
Yes, you will want to send off your torque converter and flush your trans cooler and lines.
Bill M