I decided to use some brass tubing as a guide. I'll slip one brass tube (5/16 ID) over the OD of the 5/16 aluminum tube, and will use some short, nested lengths of smaller tubing to guide the end of the reamer inside that same brass tube.

Stepped up through the reamer diameters. Note the cutting end of the reamers was not guided by anything but the ID of the aluminum tube.

OK, that worked pretty well. Not perfectly concentric at the bottom of the #3 hole, but probably within a .002 or .003.

Same approach with the tap. Using a small length of tube around the tap so that both the tap and the 5/16 aluminum tube are guided within the longer brass tube.

Tapped hole started and brass tube pulled back so you can see what's going on. This worked well enough on this scrap tube that I decided to try again on a fresh piece of stock. Fortunately I had some.

After tapping one end, I cut the tube to length with a hacksaw. I very nearly cut it too short, but it worked out.

I would have liked for the tube to be ~1/32" longer, but it's fine as is...screwing the tube all the way one direction or all the way the other still results in several threads of engagement.

The plans suggest 14 21/32" long. I would have preferred 14 3/4, but ended up with 14 23/32. As stated above, this is fine.