74 360 heads vs 72 J heads

They all (340/360 heads) with in a few cfm of each other. The best to worst differences would be 20 cfm as cast. Once you take the heads to be rebuilt (and IMO have some bowl work done,) they will be equal to each other.

When your done with any iron head (except the W2) you'll be under headed for a stroker. It's a little hard to have enough cylinder head up top a stroker. Even more so with the stock OE castings.