I bought the three points from Wesco just like this thread 1965 Coupe 3 Point Seatbelt Install

I had to drill a hole in my floor at the rear door jam, fwd of the rear seat to mount mine. They came with very large washers in the hardware kit for extra support. Some have used 3/16 plate instead for added strength.

 

Mine came with a bracket that connected to the retractor.
I'm assuming you would want to drill a hole somewhere around here.
Maybe test fit it in a location and see how the belt feels / functions.

 

I had to drill a hole and used an included large washer as the anchor.

 

Thank you, everyone. Drilled a hole through the floorboard after testing out the location and the belts are in!

 

#1 on manure instructions from those folks.
even calling the tech line was not any easier getting info although they did pick up the phone right away
i fought with it all day

I settled on what's shown in post 3 except i put it vertical

 

I found it funny when I called them and said I needed to ask a few questions about the installation of their product and the lady said they dont answer questions, that all parts are included for proper install. I said. "You dont offer tech support for your product?" She said, hold on a minute, and connected me with with someone in support, although I still had to come here for more detailed help. Now I know where Im not buying my retro radio....