I got the D-sub bracket located in position and marked where I was going to put two attachment holes. I drilled two #40 holes in the bracket and set it back in place and match drilled the holes into the radio stack frame.
I was going to use install nutplates but with the tight quarters I decided just to drill clearance holes and use a couple #6 screws with nyloc nuts. I will set the D-sub up once the wiring has been done on the connector.