‘64 Dart Three Point Seatbelt Help

Ive juste gotten three point retractable seat belts for my ‘64 Dart from Juliano’s.

The installation instructions say it needs to be inside the b pillar and welded in place, but a lot of the various installations for specific cars on their website have people sliding it up into place in the b pillar and then using retaining screws to hold it there.

I’m not sure I can get inside the b pillar, so I may need to just drill a hole for the bolt, grind down the paint on the b pillar, and then tax weld it on, and later paint over it to match.

Has anyone done this or have advice?