Upgrading wheels on 70 swinger

It really depends on what you want to run in the front for tires. With 17” wheels you could easily run 245/45/17’s up front if you wanted to, which would require a 17x8. In the back if your current tires rub on the quarters when turning then you may want to add a little more backspace to your new rims so your new 255’s will clear, but that will also depend on your current tire to spring clearance and whether you have any extra room there.

What are the specs on your current rims? Looks like they have very little backspace in the front, the wheels are almost outside the fenders.

Also keep in mind that the shorter sidewall profile means you’ll want wider rims for a given width tire compared to a 15” rim. For example, in the back if you’re always going to run 255’s you may consider running a 17x9”, because a 17x8 is the smallest rim that tire will fit on.