C6 heavy case is perferred. The case with the ribs on it. The later C6 after D7 or 1977. had problems with some earlier cases with reverse clutches and thin cases and leaking through casting, it sucked and expensive to redo. I run low gear kit, torrington roller helps on friction and taking some heat out of c6 and maybe a 1 mpg gain. TCI converters sucked and my hughes converters awesome xtm43 with upgrades. Was killing my stator in TCI and my XTM has a rollered stator and solved this problem on converter survival. I was running 40" hawgs and 4.56gears. Truck will run 130mph GPS said so! Truck pulls from 900 rpm out to 6000 easy. compression is the trick to making rpms.