With the 500 inch Caddy you really don't need an OD trans. They have so much bottom end torque you should run no more than 2.80-3.00 rear end gears and your RPM will be nice and low for cruising. A Turbo 400 is a good choice, maybe put an RV type torque converter in it to help with low end punch. That 500 should make that 57 Caddy Sweet! I was just remembering my Dad's 57 Coupe DeVille. It was no slouch with the original motor.

Pat