Finally finished my head alignment design (see posts #5,6,38 in this thread). Seems to work great, but I have not figured out the best way to test it with my equipment. I'm thinking about using my 14" B&S height gage from my surface plate, clamped to the mill table, and a dial test indicator mounted on the quill. I don't know how straight (or square to the base) the edge of the gage is, but it stands 15.5" tall.
I'd appreciate any suggestions. Here are pictures of the finished parts before and after installation.