The new pistons, also forged, are spec'd at .0035 clearance. Those 660's had no power valves, they had two speeds.
1. Go like hell.
2. Try to idle, but don't do that very long.
I put some 600 vacuum secondaries on just before I took the motor down. That ALONE was a HUGE improvement in street ability. WITH some noticable loss of power at the high end!