I've got to cut about 1-1/2 to 2" of nastiness off the bottom of the mast.[/QUOTE]
Sounds like a plan. I would cut mast first that way you know how much you have to build up the Mast step when you mill the aluminum block . Unless you replacing all the standing rigging. If you replacing the standing rigging you could re-rig to the new shorter mast height.
1972 Bristol 28