You are correct, it's 0.73. I misrembered.are you sure you have a .78 5th? By this chart they never put a .78 in a GM WC. I have a .73 in one of my T5 WC and a .68 in the other.. With my .78 5th at 75 mph with 26.65" tall tires and a 3.08 rear I am at 2000 rpm
http://www.britishv8.org/articles/borg-warner-t5-id-tags.htm