Get a piece of 1/4 by 1 inch flat stock long enough to go from the rear hole on the block to where you want the alt to be positioned forward.
Drill the block holes in it and then either weld or drill and bolt the original to that flat stock when the alt is in the right position for the pulley alignment. After that flip it over and drill the block holes all the way through from the flat stock through the original alt bracket.
