[38 coupe]

I agree that tight piston to head clearance is probably the lowest buck performance boost you can do. If you are swapping heads you can look around to see if you find a pair of EAB cast iron heads, they are the highest compression common 8BA era heads and will give a small increase compared to the stock 8BA marked heads.
Before planning any modifications just give the car a good tune up (if it needs it) and drive it as is. If it meets your needs then no need to mess with a good thing. If the motor is tired then bolt-on parts aren't going to give you great performance anyway. The car will let you know what it needs.