We never seem to hear of anyone using a Comp .650 ft, or anything like a (seemingly fairly comparable? Although I know Herb would vehemently disagree) old MP .620 in a 500+ ci combination with more modern heads.....but I'd like to.
Just to see what happens. Maybe that's an awful combination? As much as it'd be interesting to see, I'd never suggest someone else be the guinea pig.

Oddly though many folks with those sort of combinations seem to love the old MP .590 But that's almost a no brainer as it seems to fall right into the duration ballpark alot of us are thinking will definitely work.

Good luck in your quest. Dwayne won't steer you wrong and he is just cool to talk to.