g73 318 LA Motor to be rebuild.

A cast crank is fine at 500hp properly prepped.
A forged crank should handle double that if built right.
Keeping the stock heads on a 318 for anything other than stock or a mild hop up is insane. Step up to aluminum heads. You’ll need all you can get with cylinder heads w/a 4.00 arm.

What type of power are you looking to build?