Umm,
About seat belts. If Cuda's are like Challengers(I believe they would be), there is NO way the 73 rear seat belts will work in a 70. I tried!!!
That retractor doesn't fit under the seat. The fronts will work if you have the belts all from the same year. For example, don't try 70 retractors, with the 73 female end. Don't ask me how I know.
As for the rest, I have to ask if they have inspections on a car this old in your state? In Maryland, as long as it's registered as a "Historic" vehicle, meaning built before 1977, you are exempt from state inspection. If you want this car for a daily driver, then the rules will be different, I suppose.
Mike