Here's some links I saved for planned install into my '67, I am going to have the 8" sash extension added to mine so they hang closer to my shoulder as a pulling point which I have read is much better than simply swiveling at the roof which doesn't give proper restraint angle and could cause injury in a crash. Shoulder Belt Installation Tips for 65-66' Mustangs Shoulder Seat Belts in a '67 Mustang Coupe BEAMS Chrome Lift Lever Retractable Lap & Shoulder Belt BEAMS Non Retractable Lap & Shoulder Seat Belt w/Chrome Lift Lever Buckle RetroBelt::Check out our new online webstore!
You could put non retractable in pretty cheap but the retractable is a nice feature to have when you slam your car door on the buckle and bend the door...
You probably have to move the retraction unit straight back into the corner of the rear seat footwell to clear the seat sliding rear as the unit is larger than the stock one.
I really like the gotbelts ones as they look period correct with chrome buckles and no plastic covers, etc.