I have a Hughes billet 55mm tb on my M1 on my 408.

Installed the 55 centered on the m1 without a gaskets under it. Marked it with a sharpe marker, removed the tb and took the dremel to the m1 till I had it close. Reinstalled the tb and checked, remarked and took it the rest of the way. Once I was happy it fit correctly I smoothed the opening farther down so it was graduated.
Next put the gasket back under and put tb on, mark the gasket from inside the bores. Remove the gasket and cut it out with an exacto knife. (didnt mention that on my portmatch page)

http://rm-indy.com/Portmatching_M1_for_55mm_tb.htm