360 runs strange, lifters??

Have the heads rebuilt your valves are not sealing. Exhaust seats are not sealing do to the valves sinking in the seats causing the springs to lose seat pressure. You could try and shim the springs to extend the life of the engine but this would just be a temporary bandage. I know that sound very well.

The fast way to check is remove the rockers put a straight edge across the tops of the valve stems. The exhaust valve stems will be higher then the intakes due to wear from the fuel not having lead to prevent exhaust seat and valve wear . Use a feeler gauge and measure the difference on the tops of the intake valve under the straight edge.

A quick and cheap way to temporarily fix this is to air up the cylinders one at a time and throw a .020 - .025 shim under the springs with the engine still in the truck. Get new valve seals and also replace them while your in there. You'll see a big difference.