Years ago I took out the earlier "Hitler's Revenge" seat belts so that I could use the retractable 2nd generation on all 4 seats. Now, I have mounted more race oriented seat belts for the front seats with shoulder harnesses. That being said, I must also say that the car has not been on the street in over 20 years!
However, if you want to mount 3-point seat belts for the rear seats, it is very doable. If you look in the trunk under the rear parcel shelf, you will see two welded-in threaded bolts, one at each end of the shelf, for the upper shoulder belt attachment. The 'seat belt' attachment bolts can be used as is behind the lower seat, but you will need to bend the lower metal surround for the seat to accommodate the retractable belt assembly at both ends.
Why are the 3-point belts angled from the doors of the cars to the inside lap attach point? So that if there are 2 people (or 3 in the back seat) and there is a collision, they will not be thrust in the direction of each other as their upper bodies rotate around the shoulder belt - they will collide instead with the doors, seats, steering wheel, etc. For a youngster, I'd almost vote for the currently installed lap belts!
... Just my 2-cents worth.