Well the brackets are cheap enough, then when you calculate the shipping costs - wow!! I'm sure you could find some local shop that would have these or something like them - Mopac in Langley, Lordco, Overseas Auto. I need to do the same thing for my van - new seats and retractable seat belts.
In my Bus (79 and no bulkhead on either side) the existing seat belt bolt hole (center) is about 3 1/4" forward from the edge of the rear wheel well. If you want to get the seat belt retractor rearward as far as possible, I would make the bottom bracket a little longer than what you have here. It looks like it will only move the retractor back at most about 1" and might cause the retractor to hit the seat, depending on the type of seat you have. Having a longer base will also allow you to rotate the braket slightly towards the door post as well, if added clearance is needed.
I'm hoping to use either Vanagon seats or PT Cruiser seats to replace the horrible stock vinyl ones.
My thoughts.