Well - there's a few of the 70-71 SS454 Chevelles out there and if you have 30-50 grand to invest you can get a nice one. Bear in mind that a mid 60's Nova or Chevelle with a well prepared small block is really tough to beat.

500 HP is a big target and not always the mos streetable engine. You've said you "need more power.." can we ask why? Are you planning to race this car or cruise with it or a little of both?