In 1966, I believe there were optional shoulder belts, BUT they strung across the top of the front seat and anchored somewhere in the rear seat area. NOTHING like the 1970s belts!
The normal lap belt "anchors" should be in the rear floorpan "hump" rearward of the front seat. Probably just need to drill the hole for each of them?
After the 1966 model year, as in the 1970 model year, had a separate shoulder belt that was anchored and stored on the inside upper roof rail, horizontal, section of the top structure. 1972, to me, would be the better year as the shoulder belt attached to the basic retractable lap belt buckle by just sliding into it.
IF you desire to install the later version of "shoulder belts", you will need to determine where the anchor point for the upper belt will need to be and WELD the attachment and BRACE into the top roof rail structure, which will also mean a new headliner will be needed.
Might want to swap-in a 1972 C-body front seat with adjustable headrests, too?
Just some thoughts and observations,
CBODY67