I just bought a complete gen 5 502 HO for $900, it has the right heads and a complete tpi manifold. I ran the compression and was happy with 125 at 7k feet. Now the question. Does mercruiser drill through the siamesed part of the bore barrels to help cooling. There are 4, 1/8 pipe plugs about .5 inches above each freeze plug on each bank. They are not crack repairs. I was told by a local machine shop that Mercruiser did this on their Marine 502s for cooling issues. Sorry I can't post a pic.