X2 on the Holley SD. very low profile/single plane/good rep. With the heads you're gonna gain from the SCR increase going from the open 452's to closed chambered (I'm assuming) eddys in addition to the port flow. Back in the dark ages I had a medium SCR (not sure what it was exactly) 440 with 906 heads/509 cam and I advanced the cam 4 deg & milled the heads .060 or 090 (I forget which) & it was a dramatic improvement & you'll gain even more than that with your (intended) mods. If your gonna mod the dist (I would), wait till you are done cuz the cyl psi needs to be finalized