Retractable shoulder belts

I'm doing this same Mod on my 71 Duster except I'm changing from a front bench seat to bucket seats, now here comes the fabracting part, I now have to add a anchor nut for the retacters both just in front of the rear seat outboard corners and one in behind the rear side seat panel, the rear panels also has to be cut out to accomidated that retractor and cover plate, Has anyone ever done this?