I have run 4.15 stroke 400 block combos in the past, but wouldn't hesitate at all to go with a 4.25 stroke 2.2 rod journal 512. Build the motor to suit your desire for hp, and gear according to driving style. The longer the stroke in a 400, the less rpm you need for the same hp and the lighter you can build the whole rotating assembly. Don't forget the advantages of all that torque. Gear it to match. A 3.55 gear will produce about the same torque at the axles with a 512 as a 451 will with 4.10s.