D. Remley / firstname.lastname@example.org
I utilized your idea for the shoulder belt install. Thought you might be interested in technique.
There is already a vertical slot cut in the pillar that is adequate. I had to break off the tab covering it. I then slightly enlarged it horizontally to 3/4" wide. I used a 3/4" x 1/2" x 5" steel plate with a hole drilled and tapped in the center to match the Grade 8 seat belt bolt. (J.C. Whitney, seat belt w/ retractor). I also drilled and tapped (approx. 1/8") another hole in the 5" plate to match an existing threaded hole in the column (above the slot). I drilled this hole thru to give a clear thru hole in the column. I also drilled a thru hole near the top of the 5" plate and a thru hole at the top of the column. I then snaked a wire through the top column thru hole, exited the column slot, and attached the wire to the top hole of the 5" plate. I then lowered the plate into the slot, and raised it to match the 1/8" tapped hole to the 1/8" column hole and screwed the plate in place.
I then drilled a hole through the column cover plate and reattached with seat belt bolt screwing into the 5" plate tapped hole. The only trick was to make sure the distance between the two tapped holes in the plate approximately matched center of slot to thru hole in column.
At the bottom, I was able to drill sideways thru the body column at the column, floor pan level and go all the way thru with a Grade 8 bolt.
Hope this helps,