I would recommend the small chamber, small bore high risers. They call them "quench bore" or something like that, but more of a peanut shape. The "machined chambers" are huge, I don't like them. The only set I messed with had 104CC chanbers, where the quench chamber is about 65cc after CNC machining and decking. Lots easier to get good compression, even with a flat top or very small dome.
They basically call them 428HR, that is what they stamp on the exhaust side of the head. That is what Jim G's heads were and my own as well. They are hard to come by, and I think the last run was just 22 sets about 3 years ago, gone pretty quick. I have had 5 sets of BT high risers, and 3 sets of BT MR go in and out of my backyard shop in the last 3-4 years, so they are not "un-obtainable", although pretty rare. I waited 3 years for my own high risers. Jim was lucky to find a set un-touched last summer.
I would say if that is what you want, get a set if you can. Not too many high risers around, tons of MR available.
Slow 68 Cougar, soon to be slower