Full exhausts to open headers gain

I have always raced with mufflers, and my local tracks now require them (tho i cant say that i have seen them rigorously enforce the rule), so i dont have personal experience. However i agree with above, that you will need collector extensions, and 2 1/2 is WAY small for 600". I would not be a bit surprised to see you pick up two tenths/three mph, after re-jet. I've used 2"headers, 3 1/2 mufflers and exhaust on a nine second street small block (granted, at a LOT less weight than you).

Edit: if it doesnt have an h pipe, or an x pipe, could you just move them sideways and use the collector bolts to hold the exhaust up, like we did in the old days? You wouldnt have to remove the exhaust system that way. Easy a to b test that way.