451 stroker, cut the crank or block?

IMO, I would keep as much strength in the block as you can, so I would cut the crank. You already said that you were going to do work to the crank so when they have it setup in the crank grinding machine have them turn the mains down and you need to cut the counter weights 3/16 for clearence anyways. This way the block could still be used as a 400 at a later date. If need be.