I have been looking for the info I was refering to about the extreme pressure that was pushing a torgue convertor forward. I have not found the artical I was refering to, it was a extreme case like yours and there was something out of the norm causing the pressure problems.
Some info I did find though was many people having this problem (but not as extreme as yours) and the cause was a restriction in the cooler line and or cooler. One of those coolers with the temp switch valve in it was talked about causing this, some found lines themself restricted, and some coolers restricted, and some had a oneway check valve that stoped working. When the stall was flashed the pressure would spike to extreme pressures when measured at the output that feeds the cooler system. This pressure was above/higher than what was measured at the high pressure port on the tranny. They talked about rigging a gauge on the line itself just as it leaves the tranny.
I will keep looking for the info I was talking about, but this is close to it. I may even be remebering wrong and this is what I am looking for. The very artical I am looking for I can not remember exactly where it was. But there was something very distinctive about it and I will know it when I do finally see it. I was thinking there was a internal curcuit that had leaked into another curcuit to cause the high pressure, but I may be wrong. What ever the reason or cause it was so extreme that they to went through the same problems as you with instant destruction.
Anyway the info above will be something good to check & look for and just may find something for you. You do have to much already and the rest of the reason could be the cooler curcuit as meantioned. Ofcourse all engine clearances should be checked close as meantioned. But I dont think you would have the repeated failure like you are experancing unless there was something major going on. Even though you may be the only constant involved with each build (as far as engine goes), surely you would have got it close to right out of one fo the attempts and the engine would have lasted a little longer.