If you have the CJ parts available, a co-ordinated change would add some HP, but the amount would depend on the engine's current state of build.
The CJ heads have a slightly larger combustion chamber so compression would drop a bit and the valves are bigger, which would suggest the need for re-checking valve to piston clearance...more so if an aftermarket cam is being used.
My old Muscle Parts catalog doesn't show any differences in the 390's of your vintage, with the exception of minor HP differences between the regular and GT engine, which can be traced to the camshaft, with the GT having a split duration with a higher lift than the regular 390. This cam is also used in the 428 CJ, at least as a service part...
You also might try posting a query
here if you haven't already done so.
IME, asking for a 15-20% increase in HP from these changes is likely a bit optimistic...possibly, with an addtion of the CJ camshaft and some tuning refinements, maybe...
Good luck!