Not a trick question. My Cuda only has lap belts and no sign of there ever bieng 3-point belts.
OK so someone changed them out, probably long ago. All Mopar passenger cars got the 3 point system in 1974. I think you can get away with the seatbelt setup out of a '74 B-body as they look very similar. I have never measured them but I still have the set out of my '74 Charger I could check out for you.
The seatbelts are the easiest way to tell a '73 from a '74 'CUDA or Challenger without looking at the VIN
Check if you have the special rear interior panels that house the driver's and passenger's shoulder seatbelt retractors. The '74 only interior panels have little slots below the armrests that let the belt go through.
It sounds to me like you need those too.