Depends on their condition and work needed to them. If the valves aren’t sunk in, that’s really good. If the 915 heads are the 1.88 intake valve sort, you’ll do well by going to a 2.02 intake valve. The exhaust can go to a 1.65 if need be.
When you bring in the block, have it sonic checked for the wall thickness. Bore it as little as possible. See if your machinist has a boring torque hone plate. If not, find a guy that has one.
What intake manifold and carb do you have?