SOMEONE PLEASE CORRECT ME IF I'M WRONG but, I seem to remember the swirl chamber heads had a smaller chamber cc (by about 10cc????) than the earlier 318LA heads and also require a different piston profile. It's been a while though.
The change was mainly heads. I don't know what changed piston wise but you can put those 302 heads on older 318s without changing pistons. For a while it was a common swap for people to put them on 360 too.
I just bought a new set of the 86-89 .040 cast pistons on ebay for 100 bucks and that includes shipping. The gas door says ''PREMIUM RECOMMENDED'' is good enough for me