Seat Belts Again

[KFH]

I am considering putting recoil belts in the front of the car.  I am thinking of cutting an opening on the inside of the bottom of the B pillar and mounting the bottom plate there and also feeding a plate up to the top section.  This will let me get away with one opening.  What I wish to know is it possible to get a 3" x 2" x 3mm plate with a 7/16" nut welded on the back of it threaded past the metal work that supports the rear door hold open spring.  Has anybody done this?

Keith