I'm gonna do a 3 point belt also, but that's way in the future for me. Someone needs to make a DIY for the tech section. This upgrade seems essential for anyone who will have their truck as a daily driver - just for the improved safety.
***Edit*** I just did a little search on the subject. This
link describes a method for their particular belts and is for cars.
Our trucks have access through the bottom opening near the back corner of the cab - so the 1.5" hole they tell you to cut would not be necessary.
Anyway, it seems that a metal plate with a hole in it and a captured nut welded to the back of the plate would do the same thing as their special plate.