Not all the 502's came with roller cams. Some of the first ones came with flat tappet stuff. In that case you would need to get the retro-fit stuff to go roller. If it came with it then it would be easy to use the factory parts. If it came with rectangle ports then it was an HO or one of the marine variations of the HO. Many of the marine companies would swap in their own cam profile and intake and go from there. All the rest of the components stayed the same (at least that is what I had heard, I may be wrong).
Paul