Best bang for the buck in an off the shelf combo would be a 4.25" stroke with a 7.100/2.20 journal rod. If you did like I did and offset ground a used 4.15" crank (spun rod journal) to 4.30-4.315 that gets you to 517-520.

You could go 4.375" stroke but you might have to go to an external oil pump/pick-up/pan set up (some 4.375" will clear internally but it's not a guarantee every block will) but 511 cubes will certainly get it done. the larger cubes will make only slightly more power and I would put the $$$ you save on not going to an external oiling system back into the 511's Heads which would be a much better bang for the buck investment.

Using a low deck 400 (if you had one) would make a small but measurable bit more power (all else being equal) and should be a slight bit more durable. They typically wind up being 20-30 pounds lighter (not always though due to block casting variations) but they also add the benefit of a slightly lower center of gravity.

If you go in a phased approach and use a flat top piston you can upgrade heads later, just be aware that the more race oriented heads have a raised exhaust flange so when you pick headers you need to take "final phase 2") fitment into account.