[wbc409]

if you are dead set on using those belts, I am not sure that the angle iron you are using has enough surface area for the bolts and washers you are using to hold the retractors in place. I would also try to tie that mount into the factory mount by welding another piece of angle or square tubing to the angle iron. If your mount is only held in with the one bolt you could have the angle iron spin under load no matter how tight you have it.
At a minimum I would put a second bolt through the angle iron to prevent it from spinning forward in the event of an accident.