Going to agree that while maybe not the exact cause, 3.08's, high stall converter, and larger tires are likely generating a LOT of heat.
I'd really think about getting a trans temp gauge installed to make sure that you aren't overheating it.
Was your replacement converter a lockup, and is lockup working?
And how about the converter? Was it replaced after every rebuild? That should be a given as well...there is no way to clean one out, anything that went through the trans could have made it's way in there too, not to mention any damage that may have caused to it if heat was a factor in every failure. Expensive piece to replace, but I have yet to hear a rebuilder say it's a good idea to re-use a converter off of a failed trans that sent metal fragments through the system.