Thanks for all the photos and clarification Pete!Looking underneath my 71' you see these safety belt reinforcement plates. I believe for adding rear passenger safety belts, but if I was racing her (nope, never going to happen) I'd remove the passenger side one and relocate it on the drivers side for the second shoulder belt to attach too. The other shoulder belt can probably connect to the one that Alfa installed ... but it might need to move in a little?
These attachment points are designed so that the safety belt goes through the rear seat where the base and the backrest meet, so no holes in the rear seats are required. Most cars do the same, for rear seats anyway.
Sadly Renzo does not have these attachments.
-tj in the Cruz Mtns