You have to make sure that you are good at welding or get someone who is to do it, if it's not welded properly it could be lethal.
While I'm not encouraging a poor welding job or bodge... please keep in mind that even a poorly installed pair of rear belts offers at least the chance of SOME protection. In the current situation (as I understand it) there are NO rear belts at all in which case there is NO protection. A poor installation job may create a false sense of security but it is not going to be any more lethal than no belts at all.
Aaron, if you look at the second picture and refer to my previous post, I mentioned that the ribbing on the seat belt stamping prevented me from making vertical plates for the outer anchors as was used in the Mayfair. Instead I made a pair of LONG horizontal plates out of 1/4" steel, welded nuts to them, and attached these to the bottom outer corners of the seat back. That's the detail I was trying to show in the bottom half of the second photograph. You can see the nut on the plate tucked into the corner behind the fuel tank and battery.