Roachy,
Some options for you:
a) Frankenstein the bolts through the B-pillar. This can be ugly (bolt heads on the outside), though is common. I can see you already know that the UNF bolt head is small, with little surface area under that bolt head actually holding the load in place during an accident. A 7/16"UNF bolt head (a.k.a Frankenstein bolts) have a surface area of just 235mm2... around 6% of the minimum area NCOP recommends for reinforcing plates. Plates behind the pillar (regardless of shape) will hold 16 times the load that the Frankenstein bolts will.
If you do refit the bolts, you must either fit anti-crush tubes or used bolts that prevent crush of the pillar by design (they bottom out before crushing the pillar).
b) You can cut a slot in the inner B-pillar skin and lower the flat plates down. The slot is normally welded back up. Not so good for you with new paint (I had the same drama).
c) The method I used was a little different. I ended up using a hole saw to cut a hole (26mm from memory) into the B-pillar (inside the car) at about shoulder height. I used a reinforcing plate of around 6"x1" (correct mm2 size) with a captive nut welded on the back. The plate was a cad-plated off-the-shelf job from the NSW seatbelt Inspection Station. I drilled two 1/8" holes in the reinforcing plate, one at either end, and made similar holes in the B-pillar (one above the 23mm hole, one below). I put a piece of fishing line on the plate and dropped it into the hole (catch it with the fishing line). Use the fishing line to pull it up, then put the seatbelt bolt into the plate (can't fall down now). Hold it in place with the bolt, and put a pop-rivet through the B-pillar hole into the plate. Repeat the pop-rivet for the other hole. You end up with a hole in the pillar with the plate behind it - can cover the hole with a plastic cap, though the seatbelt top "pulley" bit covers most of the hole once it is bolted in place. Seems a bit funny leaving a hole there, but the surface area of the plate is a heap bigger than the surface area of the bolt-head used in "Frankenstein" set-ups (much less likely to pull through in an impact). This is the same process reccomended by one US seatbelt manufacturer to refit old cars. This is the US seatbelt supplier I was referring to in the previous post:
http://www.wescoperformance.com/3-point-seat-belts-shoulder-mt-door.htmlNote they use a slightly different plate, and hole-saw in a slightly different place.
In any case, not a bad idea to check with your certifier (engineer, seatbelt inspection station or just your own judgment) before choosing a method. Some of them (eg engineers) get very toey about the location of bolts and sash guides.
Cheers,
Harv