go 572. or better yet even more.
see HEMIVIPERS triple black 71 charger listed in the B bodies for sale section.this was my buddies old car.
612ci runs on pump gas, low gears and LOTS of torque so the low gears won't slow you down. when my friend had 2.76 gears in the rear i think he got over 10 on the highway with the A/C on. and that's with a dominator carb.

perception is 90% of reality