If you don't have a problem with the manifolds just leave them as they are. Consider using a Remflex manifold gasket, which is a thick compressable single gasket that can take up to 1/8 inch gap. They are somewhat pricey at around $40 but that is a lot less expensive than having a machine shop mill that long manifold.
Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix