What did the car the motor came out of weigh, and what was the speed?
IMO not a converter builder, but it seems to me that in order for an 8" converter to be loosened up enough to stall 7000rpm behind a motor with less than 600ft/lbs, it would lose some effeciency in the process.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads