J (915) vs. Magnum heads

Agreed.

J's are open.

The T/A cars had the pushrod holes moved. Those would be ideal, but you'd need the rockers that go with. Also, if you have those heads, start looking for the rest of the car.

If you start tinkering with cylinder heads, look into a set of Engine Quest heads to start with. Your parts will swap into them and they are better castings than stock magnums. Or look into a set of aluminum heads.

The only way you'll see a gain on a set of J heads out of the box is if you are running negative cc pistons (positive deck height) like the T/A cars ran.

Have you checked cylinder pressure?