I used a 3/8 bit like @imyahucklbrry and @A10Driver suggested and it fit perfect. I just used my cordless drill, since I don't have a drill press, but wasn't an issue. Then used the angle grinder to cut off the ~4" of excess bolt before putting the flip kit on.
Thanks @wngrog for explaining why...
Trying to imitate what @wngrog has done on most all of his builds, I ordered steel shims from Georg at Valley Hybrids and the 4Plus U Bolt flip kit. I’m trying to install now.
I got the old U bolts off, and I’m trying to put the shims on, but the long bolt that came with the shims doesn’t...