I drilled the first hole(s) to hold the ejector rod. I started by using the 5/8's bit as a spud to line up the bore and clamp the block in the X,Y vise so the angle would be correct. I want the hole parallel to the flats. Personally, I've found drill bits wander so I started with a #29....it was a new bit, and much smaller than 3/16's I'll work up to. True to form, the bit came out a few thousands to the left. I remounted the block, muzzle end up, and used a center drill to 'warble' a tad to the right so as to drill the next size up. I'll drill another size hole larger before finally reaming to 3/16's.
I might add at this point I've inserted some brass shims in the 'T' slot to hold the lumps and block tightly in their correct position. I will not braze them until the blocks are threaded and barrels fairly well mounted. Using a 5/8th's rod in the 'bore' and a laser, it's amazing how much a couple thousands movement changes the line!