I purchased a custom built .40/65 and it does not have a very good rear tang sight. The mounting holes for the base of the sight that is on there looks to be 1.550" apart. I would like to buy a mid range sight that I can just screw back on without going to the trouble of drilling and tapping. I compete so I want a halfway decent sight (not much over $250 ) but have no idea how to find one with proper spacing. Any ideas ?.