This is a typical problem with SBC starters......They need shimming! Get a shim pack for it and experiment with different ones until the noise goes away.
Otherwise, I have had to grind on the mounting surface to get the teeth to mesh without noise.
GM should not have gotten away from the old three bolt starters that attached to the bellhousing.
Are you using the brace on the front of the starter (it attaches to one of the "long" bolts on the starter and goes up to the block. It prevents the starter nose from "warping" or twisting out of shape.
If you must have a two-bolt starter, try to find one with a cast iron nose. Much better
