http://johnsmustang.com/Catalog/4138/Those plates are made to back up the seat belt bolts. I don't think there is an exact standard location as they were a dealer installed option IIRC.
With the seat bottom out lay belts in the position you believe will be closest to correct. the inner belts usually mount right at the base of the transmission tunnel. Before you do any drilling, take measurements and get under the car and make sure you not hit anything when you drill through and that you have relatively flat floor pan to mount to. your 1st drilled hole should be 1/8th or so to be able to insure that you have located things appropriately. Then move up to the fastener size required by the belts using Grade 8 bolts. Clean the underside with a flap wheel or the equivalent to allow proper welding of the mounting plates. Tack the plates in place. Make sure and paint over after.
There may be some folks that think full perimeter welding is a better idea, but I do not. I believe the plate sandwiched to the floor pan is a strong enough anchor. When you weld sheet metal, you weaken the metal around the weld to some degree. For this reason I do not recommend welding all the way around.
I do not know if the new structural body adhesives are adequate for this installation, but would be inclined to try them myself.
