How much difference would 19 cubic inches really make going from 451 to 470? My 3.75 crank is a new offshore 4340 that measures OK. But if I only go to 3.91 I don't know that it would be worth the money for new rods and pistons with only that much offset grind.
If you have all the parts for the 451, rods pistons, etc stay with that.
