I drew up the mounting plates and tabs in CAD and had them cut on Midway Iron's CNC plasma table.
After measuring carefully and drilling a couple of mounting holes, the plates are in place on the B-pillars.
1.25" OD .120-wall DOM tubing.
Not pictured are the threaded bungs, which slip into each end and get welded in place. I used this same tubing for the downtubes, despite it being way overkill. I didn't take any photos of the downtubes - they're just a straight shot of DOM tubing with a right-hand bung on one end and a left-hand bung on the other, to allow height adjustment by turning the tube.
Harness strap loops bent up and trimmed.
Ready for paint.
(Despite the bar appearing too high, the belts are horizontal when I'm seated.)
(Excuse all the yellow foam; I was in the process of reupholstering the rear bench to match the buckets.)
Suspenders! herpty derp
Since all the attachment points for the harness bar are the original seatbelt bolt holes, the whole shebang could be removed without a trace. The only modification to the car was cutting a sliver of plastic from the back of each B-pillar trim panel in order to provide clearance for the mounting plates.