Tweety

I have recently purchased a 1972 cutlass, and I am looking to get a set of adapters made so that I can install a set of 16x8 trans am wheels that I happen to have. The problem is that the wheels currently installed on the car look like SSIII's, they are body color matched with the chrome trim and 2 have the snap on centers. The only thing is, I measured the Back spacing on them, and it measured to be 3.5". Now I was told there where no olds wheels with that BS. They are 14x7's. I also know that they are not the original wheels that came with the car. My question is, when I am figuring for the adapters, should I use 4.5" or 4" as opposed to 3.5"?