Tom- The 12 inch converter sounds about right to me. Most 12 inchers stall around 2500 on big block engines. If a 12 inch converter stalls any higher they tend to slip above 5500 rpms, and this will build up heat and cause converter/tranny life to shorten. To stall any higher and not slip on the big end you must go to a smaller converter. Heat generated by slippage is the bad guy here. Ideally you want the the converters stall to be less than the rpm range you cruise around at. That way you keep things nice and cool. I also feel that a tranny that has a "loose" converter should have a tranny cooler to. In fact, I feel it's almost mandantory for street performance cars anyway. As for the C-6,
I would use it if for no other reason than you have it allready. Just make sure it'a a good un' though. Using a C-4 for performance could make a good subject for another thread, as there is some advantages AND disadvantages to going this way.
cobrashock
