IF you have it angle milled, the guy doing it can easily spot face and redrill (at the same angle) the mounting holes.
I actually did something similar to the way you originally asked about. I milled and drilled some angled spacers for a set of custom headers..... but it only worked because the headers had the Stahl mounting pattern. Spacers bolted to the head, headers bolted to the spacers.
I don't see how that could work with stock manifolds.