In doing some research on the rims and tires needed for the MattMobile, I kept hearing the term "5 on 5" when referring to the Caprice Classic. I dug around, and found this website that explains it:
Long story short, the first 5 is the number of bolts on the wheel. The second number is the diameter in inches of the circle that the bolts make.
Also, for reference, this project has been officially dubbed "The 89 MattMobile". Huge thanks to RPF user E-Wan for suggesting it.