Retractable shoulder belt installation?

[Hogtrough]

I've installed retractable shoulder belts on every Studebaker I've owned- but this is my first Avanti and the belts will obviously have to be bolted to the roll bar. Normally I drop a threaded plate made for the purpose down inside the door pillar. Anyone done this on an Avanti, and if so, how?

[brad]

I used a retractor installed hidden under the rear armrest ashtray bolted to the roll bar. Up from there, anchored to the upper roll bar, then the free end anchored to the floor in the stock outer anchor location on the hog trough. Inner latch buckle to the inner regular seatbelt mounts near the driveshaft tunnel.

[Hogtrough]

How did you anchor to the upper roll bar? Drill and tap for a bolt (since the stamped steel is fairly thick?)