On my 539, I run the Dart 320 heads along with Ross pistons. I had to cut "fire slots" in my pistons, run the Autolite race plugs, and index to keep from closing up the gaps. I have two different sets of the Dart heads and one set uses the gasket seat plug AR3933, and I have also used the NGK plug R5671A-8 (stock #4554). But the other set of heads use the tapered seat plugs and I can't remember the numbers, right now.
Bill M