Definitely go with hardcoat anodizing. The wear resistance of the anodized surface finish is a lot greater than any paint will be. One dunk in the old acid bath shouldn't make your holes much bigger. It will definitely not dissolve enough metal to create noticeable slop between the upper and lower. In fact, anodizing grows a finish which should actually make your holes even tighter. Your parts will "grow" larger by about .0005" even with the home-brew anodizing. If you send your piece out for the even thicker type III finish, your fit will become even tighter.
Someone correct me if I'm wrong, but if I recall correctly, the holes you drilled in your receiver are slightly oversized to compensate for the extra .0005" or so, an anodizing will give you. I might be smoking crack, but I swear I read that somewhere.
PS. Guncoat will probably clog your holes and threading. Go with Norrel's flat black if your dye job is so ugly you gotta paint the thing. Either that, or plug holes and mask threads prior to Guncoating.