I kind of like your plan with the 596 heads and the RV cam. The 318 heads might not hurt you much--hell, they'd help in the bottom-end grunt department--but the 596s are done. Any money thrown at the stock heads is money wasted in my mind, and you admit the guides are wasted.
If the truck is set up right, the 318 will pull without a problem. In fact, a slant six can and will. Just don't ask them to pass 3 cars at once going up hill trying to hit 90 mph. Keep the expectations real, drive it as it would have been driven in '77 standards, not in 2025 standards.
No delusions there. The Army dash tag says "Do not exceed 60 mph" LOL. It'll be turning 2800 @ 65 on the highway. Can't say I'd want to drive it any faster.