Stock exhaust manifolds vs headers

600cfm is kind of large. Whatever happened to the 390 Holley's?

I used to run consistent 10.58's in the 1/8th with 3 on the tree, 198 open 3:23's, M-1 intake 318 BB or a Holley 2bbl off a slant. Stock valves and cam, Hooker headers with a H pipe crossover duals out the back, with Superior round turbo mufflers. Head has .090 off it, stock valves lots of bowl work, porting, polished chambers.