hard to say which is harder, but since you have the barrel mounted you will need a bottoming tap to clean the front two holes. if you think a 6-48 is hard to find, a bottoming 6-48 is going to be fun. go slow, and get a tap handle. but a set and use the handle then return them if you have to. just watch how it goes in. you may get some of the gunk out by running the screws in and out. i did that and it was easier. until the screws had to come out, then i stripped out the heads on a few.

Tap. Torx. Ta-daa!