A 3/8" reamer better be OK. That is what I used. If it made a difference, then the plans should not say 3/8" reamer.
Gil. If using the .374 reamer for the #6, what size
should the pilot bit be in the final cut before reaming?
The plans have us drill to 11/32, then ream to 3/8 (9.5mm); so by the plans the drill is 1/32 under. 11/32 is 0.344.
For the 3/8, that is 0.031 under, and for the 9.5 mm that is 0.030 under. I don't think my drill press is perfect though. It probably is slightly off, but I can't tell how much.