I haven't been watching flow specs of many Cleveland heads over the years, though I've gotten an idea of how the CHI heads flow.
So IMO, I think the key is having enough airflow in the heads and intake, to reach the power levels we hear about very rarely. The stock heads need a lot of work to kind of correct the massive port sizes, and exhaust flow angles. Most people aren't ready to do that, stuff which used to be trick and rare in 1980, now it's tough to find a shop that can do it.
The aftermarket Cleveland heads all seem to be aimed at mild performance, most flow well under 300cfm. I'd bet they are all very close to the level of the tons of common Windsor hear choices, which run about $1500.
So I think if a big number like 600hp NA is the goal, you won't get there with a 270cfm head. It probably will take a head that's over 300cfm, and that leaves a short list of heads. I know many push to 400cfm, but those are $4k heads etc.
I was hoping to find a CHI 3V head that flowed near 300cfm(@.550 or .600), for a NA 351 Clevor. They want you to buy the little 185cc version, yet they make a 208cc 3V head, and one a little bigger still. I like the CHI heads because you can get them with Windsor end bolt patterns. I'd mount stock Explorer 302 brackets and accessories to the, all common and easy to get parts, for a Windsor 302/351 block.
@damage, your plans will include bigger cubes I hope, so add the higher compression and custom cam. Use a big intake, and big headers etc, and maybe you will be close depending on the rpm's.
I suggest asking directly some of the old school Cleveland builders. There are some here who know how to cam them, most know the OEM heads well, and hopefully someone knows how some of the aftermarket heads work.