My 74 had only lap belts and I converted to shoulder belts. Several companies make inertia locking belts. The key is to get the belt mounted high enough, about the height of the seat itself, so that you don't get forced down in an accident.
On my car there are four factory threaded holes in the rear passenger compartment. Two in the floor under the "seat" and two more on the inner fenders. I designed this bracket to mount to them, one on each side, mirror image. A 7/16" rod connects the bracket to the floor hole. The brackets are connected to each other via a 1/2" all threaded rod.
I don't have pictures of the installation, and my car is currently at a shop getting some paint work corrected. But, the system works very well for me.