It is a Shimano BB UN 73 square taper with a plastic non drive side ring/cup. At first install on new frame, the plastic seemed warped and would not go all the way in leaving a couple of mms on both drive and non-drive sides. I installed it on my road bike and it looked fine so it is *most likely* not warped and a possible problem with the new frame.

What would be better/cheaper between getting the threads tapped and cut deeper on the new frame or just getting a spacer, like those SS/fixie folk use to achieve a perfect chainline, to just cover the gap? I am leaning towards the spacer since that probably is cheaper and my girlfriend might not want to spend any more money for her bike so I would pay for it. Or could I just leave it. The gap isn't huge, but there are some threads exposed on either side and to me, that isn't a good way to set up a bike - operable, but just sloppy.

Thanks for any input.