If you want "mandatory" shoulder belts in the front you'll have to either go with an aftermarket set-up...Lots on EBay, or look for factory 1974 Cuda/Challenger belts, involves cutting your rear interior panels and I'm not certain about the anchor points.
If you want "optional" shoulder belts in the front...any year Cuda/challenger up to 73 will work. You can get real nice new reproductions of the 70 or 71 belts, but they are all made in China.