I have simular, though slightly lower HP expectations (650-700 full tilt on alky). Well changing the flange is no big deal for me, I have poston's steel headers for just that reason since I'm already going to have to modify them to clear the '64 Riviera frame, steering linkages, and oilpan in addition to adding the flanges for the deltagates. However cost is a factor, the cheapest I can find a GT40R is around $1600, what is the PT67 R-Trim BB going to run?
The PT67 P-trim BB is #1374.99 Plus $60.00 for the water line kit.
Pete