I used a cordless drill and Vermont American (brand?) 1" grinding stone and cutting oil to open up the hole to almost an inch.
Then I built a piloted reamer setup, which is now in circulation with Bravo squad. It is comprised of two shell reamers (1 1/16 and 1 and 1 1/8) that I scored on eBay for $5 each, a threaded 5/8 rod, and about 6 inches of 1 inch diameter copper pipe with an adapter silver-soldered into each end. I ran the smaller shell reamer through the rear tower with the upper clamped slightly forward of its usual position, rear takedown lug inserted into the fire control area. Then repeated the process with the larger reamer.
To tap the hole, I clamped the lower onto the base of my drill press with the rear tower pointed up. I set the tap into the BTH as straight as possible, lots of leveling and measuring involved. I chucked a small piece of 1/4 inch steel rod with a point ground onto it into the drill press and centered the point on the small dent in the top of the tap. I held pressure on the tap with the drill press handle and turned the tap with a crescent wrench. The method is pretty well shown on the Builders Squad Project site. Worked great for me. Oh yeah, I drilled out the buffer tube retainer spring/pin hole before I tapped the BTH.
Far and away one of the most enjoyable projects I've ever completed.