J (915) vs. Magnum heads

I think both heads are good if you already have them. as mentioned above there's a lot of things need changing when swapping from magnum to LA or vice versa.

Different intake manifold. Different valves, retainers and keepers. Different rocker covers. Different rocker design. I think maybe the springs can be transferred from one to the other and that's about it.

If you're after increased performance, there are more productive ways to spend your money.