i have a strange sight combo on my 75b and ran into exactly what the op said.
got the competition rear sight from Dawson and had to remove a fair amount of material from the bottom of the sight (behind the dovetail) so it would clear the top splines of the slide.
It worked, and the sights are great, but it took a fair amount of sanding/fitting
I wish I knew how to post pics, I'd show ya.