They were used as 350 heads from '76-78. Look at the same chart. Yes, they were also used on a couple of 400s, most of the 400s got 6X-8.

If they measure out at 87cc they should be okay with premium fuel with a standard bore 455. If he ends up boring it or if the heads have been milled he's going to be pushing closer to 10:1 again. Maybe that's what he wants, but even a stock 455 will haul a Firebird around pretty well.

This is the fourth set of heads he's bought for this project. Dunno about you, but I find it cheaper to plan first then buy.