Hi SS, thanks for responding. I have been doing alot of research and I have found that exact thing. The Red Eagles or Blue Plates with full thickness steels hold up better under heavy use. I know that the guys who built this trans in 1999 found some way to get 3rd gear to stay together. well enough that it lasted this long. I will find out what they did when I tear it down in the next few weeks.
In my research I have learned that there may be no lasting method for building the 3-4 clutch using the factory input drum. The backing plate just flexes way too much and under high pressure, the snap ring can pop out. Sonnax offers a redesigned part called the "Smart Tech" input drum that does away with the snap ring and flimsy backing plate, replacing it with a much more sturdy and flex free bolt on backing plate. The part sells for $500 so it's not a cheap fix but it seems like it will hold up alot longer. Have you checked out the PATC website? Also, how hard is it swap out the turbine shaft from one input drum to another? It seems like it might be tough. I never even considered it a possibility before. The Smart Tech comes without a turbine shaft.