i haven't completely read all the replies and i'm not going against any of them but i don't think 600 is all that hard to get.

the 451 in the sig car has home ported RPMs, ross FT pistons, around 11.5:1 CR, .585/.585-275°/275° flat tappet, HS 1.5 rockers and an M1 4150 intake with a 950 HP carb. car weighs around 1800# with me in it and runs 160 mph all the time (best of 162) so it has to be close to 600 HP.

not everyone has the budget to make changes but imho i would NOT use that manifold, those rockers or those heads in that order. unless there's some reason you really want or have to use that manifold get rid of it pronto.

you say this is a drag car so you don't have to worry about constraints you would with a street car

also if you have a performance target in mind remember there are a lot of other parts surrounding the engine that should not be overlooked