I ran 18 and 20" billet rallyes on my old 69. But its been so long i forgot the specs. Im always buying three piece wheels not meant for my specific car and changing the inner or outer barrels to alter the backspacing/offset. The best advice i can give you is to learn how to measure wheels. Backspace is really simple. Here is an offset calculator you can use to compare your current wheels to the new ones. You have to convert your backspace into a mm offset though. Backspace is simply the measurement from the back edge of the wheel to the wheel pad. And offset is a mm + or - from the wheels Center line to the wheel pad. Backspace is easy to measure. Offset you calculate.
Heres the backspace to offset calculator
https://www.customwheeloffset.com/ti...et-calculators
And heres the calculator to plug in your current wheel specs and new wheel specs and it tells you exact how much the inner and outer edge of the wheel sits compared to your old set up
http://www.1010tires.com/Tools/Wheel-Offset-Calculator