Sorry I got away from this thread for so long.
If I was going to use the counterbores, this is how I would do it.
Buy the 5/8" pilot and a 3/4 and 7/8 cutter.
Drill the barrel 5/8" all the way through, ream if you do that.
Mount the 7/8" cutter and cut to depth, leaving the hole in the center still @ 5/8
Mount the 3/4" cutter and bore to depth
Should be a fairly easy gig, and you can find these cutters in reduced shanks if you look around.
Just do a quick search for pilot counterbores on the internet. Try Victor Machine, that's where I buy alot of my stuff.
Bob