Louis,
.020 should be fine without cutting the intake or the intake side of the heads, but I consider this to be the max without cutting the intake side of the heads or the manifold and still make it bolt up and work. At .030 I cut .030 off the intake side of the heads and this makes the heads seem like they have been trued up with a .005-.010 cut and things still line up and work like they should. Also on your heads they use a 5/16 bolt and have 3/8 or larger holes so the only thing that would concern me is a minor miss alignment of the ports. But most people don't get the gaskets aligned right anyway so it's not much of a problem. Heck the factory miss' by more than that stock. IMO I would go for it, but check the intake for fitment before bolting it all together. At the end rails and intake surfaces. I think that you'll be OK.
