TH 350 Problem

[Chevy 350 Fan]

Hey guys I'm a senior in high school. I have a 72 GMC C20 350/TH350. Trans was rebuilt last fall. Dropped it and reinstalled myself. It was built by AAMCO. I always babied it along as it is my baby . Well in March the truck decided to catch on fire when I was driving it to school for the first time this season. Well I rebuilt the truck and replaced just about everything. I got my hands on a 450HP corvette 350 motor . Well I decided I wasn't going to buy a new trans because it was just rebuilt so I figured I run it until it dies then get a nice built one from Monster Transmission. I also decided to stay with the stock stall converter to make it a little easier on the trans (not as hard of a launch). Consequently, I have to have the idle set at like 1500-1600 otherwise it dies when in gear (cammed engine probs). Anyhow I recall when I drove it for the first time it was pretty sketch because it was far from being finished. I didn't have a vacuum line running to the modulator so I just used L1 and L2 to go around the block. But if I remember right "D" always worked fine it just didn't shift out of first because there want a vacuum line to the modulator and I'm not going 45-50MPH around the block to make it shift. Well now it will not into D. But reverse and low 1 and 2 are fine still. When you shift into D it makes a terrible noise and it sounds like something is spinning and possibly thrashing around in the trans but there is no power to the wheel and the engine RPM's are as if there is no load. If I go from D to reverse it makes another bad noise and SLAMS into reverse. Probably because what ever is spinning is suddenly stopped and must spin the other way. IDK tho!!!!! I also recall like the second time I drove it or moved it in/out of the garage it would sometimes be fussy about going into D. But I would always be able to make it to move. However, now I'm questioning myself if I was just putting it in low. Also when you go from D to park and it making that bad noise it will continues for a few seconds even though it is in park. I did some research and someone said his grandpa told him if it pops out of gear hit the brake and giver her some gas as dirt can clog the valve body and this gets it out. and when it does it will slam into gear. Another post i saw mentioned the roller clutch not engaging. I'm not sure what I can do about that without dropping it and having it rebuilt. I called AAMCO they weren't much help. They told m my forward clutches were fried. Then I told them they built it last year and then he got defensive and thought I was mudding and blew it up. Well sir its a 2WD truck and I always babied it. Plus I'm running a big AUX cooler. He seemed to think something broke internally. Well all I know it I was cruising along fine with it before the fire at 50MPH. I don't think the fire really affected the trans at all because the plastic vent is still intact so that why I decided it was probably ok to reuse. ANY advice would be much appreciated!!!! I was hoping to get some life out of that rebuilt tranny but if not then I guess I'm going to be fessing up $1500+ for a trans from monster, and a higher stall converter haha.