Transfer the RA23 seat rails to your 'NEW' seats, and use the rail bracket to mount the belt.
Cant remember if 23's have "left over holes in the floor", as the TA22 mounts the seat belts on the tunnel. Could well have the holes, as firewall forward, was the major change between TA and RA.
If no holes, drill a couple, and go get a couple of "Child restraint mounting kits" These have a plate with a captive nut, which you would use as the anchor for your seatbelt.
Or use 1.5" x 1.5" x .125" steel plate,which can be used in body panel to mount seatbelt.