I would mic the bbl and the ID on the gas block. Someone may have reamed the gas block in the past. If so they are pretty cheap to replace or if you are wanting to keep it all matching once it is pinned on it will be fine however getting it to stay in place will be a challenge when drilling. You can use a C - clamp and padding or take some pipe and make one the exact length from gas black to rear sight and then one from front of gas block to threaded part and cinch down to hold in place.