It sounds like they are the old Powerpack type heads. Casting #s 896, 520, etc. They are probably small (1.72" intake) valve heads. I have seen them opened up to 1.94 valves. They do not have hardened seats.
I've got a couple sets that I've never done anything with. My machine shop said they "run out of breath", but I remember high-winding 283s.
On a 350, they would give high velocity and be good on the low end.