It's probably not worth putting $ in the old block given the crack and 60 over. I would park the block and leave it for the future tech to fix it.

Have you put money into this 75 engine yet? There are built and rebuildable 67-69400 engines still available. I know of a 68 400 ready for a rebuild right now. It's for sale now I believe.

You need to properly design your build or expect trouble. You have to decide how your going to handle today's gas in your new engine as well as other issues.