That changes everything. Use the 500. The compression is perfect for an aluminum head street engine. You need to find out what cam is in it. It may be OK to use. What manifold is on it? Chances are it is OK also. The rockers will be OK as long as they are not some inexpensive Chinese junk. If they are then they wouldn't have held up in a race motor either. As far as the pushrods go, he probably wants to check the length with what ever cam would be put in.

IMO you already have the right engine sitting there ready to use unless the cam is way too big.

Should be way past your Horsepower goal even with a mild street cam.