Are the heads ported ? If you have have that big of cam in the 280s at .050 I'd guess you will hit 660-700 hp is range at 6300 rpm.
Just for a reference I have 264 @ .050 in my 470 and make 675 HP at 5800 rpm
Man I hope it makes that. I was going to have him port them to max wedge size but the money got tight. I might get them done when I go to a bigger cam later if I ever do.