While the situation isn't ideal, given the so-called "professional" reputation of the rebuilder, it also sounds like you're turning a minor annoyance into a tempest in a teacup. There are probably dozens, if not hundreds of mechanics and/or machinists in your area who would be willing to spend 15 minutes plugging that hole if you're not confident in your own abilities. I wouldn't even bother threading the hole- - - -I'd just turn an aluminum or brass plug a few thousandths larger then the diameter of the hole, drive it in with a hammer and a punch, and draw file it flush with the top of the block. If there isn't a corresponding hole in the cylinder head or the head gasket, leaving the hole open shouldn't be a problem.
Jerry
