I didn't overly worry myself with the accuracy here. I had the points marked out, and did still get the drill lined up very well, but the holes were to be oversized, so some play was fine. I need a hole right through, so I could feed in a socket head cap screw, with which to screw this into the mill's vertical column block, with the front hole being bigger, to accommodate inserting the SHCS in the first place. For the first hole, I drilled a large hole in the front, then switched to the small bit, and fed through that to cut the small hole on the opposite side. For the second, I drilled the small hole through both with the bit still in the chuck, and then switched back to the big one to widen the top hole only. This was the worse method. The bit had no center point with which to register, and chattered a bit, creating an ugly larger hole, whereas the first set of holes were both pretty perfect. And that's what it sounds like to gain experience. It's nice drilling a series of holes down a long, straight part in a vice, because once you align the first, you just slide the part down, and you know that one dimension will remain aligned by the jaws.

I couldn't get the broken tap out of the old aluminum block, despite using adjustable pliers, and a file. It was in there much too firmly, and was too hard to file into. Instead, I just grabbed another 10-32 tap (I have a stock of taps on tap :), and another block, as I have a pile of about 20 of these little scrap chunks. I filed its mangled edges smooth, drilled the 2 through-holes again, and set about tapping things the proper way: 1) drip a bit of tapping fluid on the tap 2) twist in until tight 3) twist out 4) brush chips off tap 5) blow chips out of throug-hole 6) ??? 7) profit! All four holes came out perfectly, and took only about 10 minutes each, and about 8 twist-in/twist-out cycles. Here the larger, untapped through-holes have socket-head cap screws holding the blocks tightly to the base plate, and the threaded holes are ready for the hold-down clamps.