If you go to
Classic Wheels and then Offset Chart you get a useful diagram with the necessary measurements.
I am no expert and I had an unfortunate experience with another set of wheels after using measurements that I derived from other owners of Harrisons. They all vary in the way that the chassis sits on the frame (L-R) and the type of suspension fitted (typically R32 or R33) and also with how the suspension is set up.
On the chart (above, but I will send this by email) measure all four wheels carefully with a ruler and straight edge and determine the required measurements that keep the eventual tyre (your 18"s) within the 'guard edge and not hitting any suspension or brake components on the inside. Take your time with this step as it is the most important. Then talk to Dragway or another provider.
For my 17"x 8" rims, I eventually used a front space of 78mm on the Front and for the 17' x 9.5" on the Rear, 94mm. Fit is perfect, even with my larger AP calipers and disks. You can determine backspace as well using the diagram for a known rim width.
Not sure what bolt pattern that you have but they may vary from front to rear on some Harrisons (GM/Ford/Nissan).