QUOTE(badm @ Nov 29 2010, 07:41 AM)
Please don't!!!!!!!! There is a reason that soft steel and corse threads are not used on seat belts mounts, do you want to trust your life to something to made to hold a 2x4 down?
The square U-bolts (3/8" thread, mfg unspecified) shown on the McMaster site list a working (not ultimate) strength of 1000 lbs. Each.
Anything like this will, I suspect, be ultimately limited to the strength of the firewall if that's what the're bolted to. (If they're part of the cage, you're probably using s different technique for the belt guides to start with.)
Back-of-the-envelope - 2000 lb strength, 100 lb torso, 10g impact.
I think 3/8" is too small for my comfort, but 7/16" might be OK. I'd want to see the specs.
The rear axle and leaf spring mounts on my half-ton truck are in the half-inch range and I promise you they see more than 2000 lb loads on a regular basis.
This is not a recommendation, just a suggestion to consider the loads.