If there is any passage way, up thru or sideways, into the desired threaded plate location, just weld it to a small rod or tape to a stick. Locate it, with this device, to be centered in the drilled clearance hole. Place the upper shoulder bracket in place and start the bolt into the threaded plate. If the plate is a rectangle or triangle it will rotate and stop when it hits the side of pillar/frame and then you can completely tighten the bolt. Rip loose the taped on stick or abandon the welded rod. It also may be possible to hang the plate with string from above. Some installation instructions say cut a slot in pillar to slide plate into. Remember to consider that on old trucks, the seats are not designed for crash. So mounting to a seat bolt is possibly not advisable. But also consider, if belt mounted to floor, that the seat may come loose and the seat belt may stay bolted to floor and cause injury when you and the heavy seat move forward. It's good to read up on seat belt installation guides, recommendations and theories.
I don't think there is anyone who would rely on bonding in pull-out mode but to hold a plate in place until bolting is no problem. But letting it rotate to contact pillar is what you want. In a crash, if it breaks bond and rotates, that would NOT affect holding. The rivet or bond is to hold so as to tighten bolt during initial assy. It's mush easier and less holes to fish it in and grab with bolt and let it rotate to stop. Round off the corners a bit on plate so it does not dig into pillar. You want a "wedgie" effect.
