With 300 plus 4-40 holes to drill and tap, and a timely "for noobs" post from That Old Dog Ray, I whittled up a couple of tap guides out of wood closet pole material. They're ugly but they worked.
Never broke a tap and as a matter of fact I did 'em all on a single tap. Thought it might wear the tap out along the way but it was still cutting good threads at the end.
If you was nearby Ray, I'd buy ya a beer!