Depends on which driving games you want to play.
Older games where the wheel spun all the way around (like pole position) used an optical encoder and would connect to the opti-pac like a spinner.
The optipac has connections for two buttons, but they will be seen as mouse buttons. You'd probably be better served by running an ipac for the buttons.
If you're only interested in the early 80's games, that should be all you need unless you plan on having analogue brake & gas pedals.
More modern games that were limited to 270 degrees (i.e. crusin usa) used a potentiometer. The potentiometer would need to be connected to a U-HID or similar device that can read analogue input. Another option would be to hack a pc steering wheel like done here:
http://forum.arcadecontrols.com/index.php?topic=105186.0One type of game doesn't play well with the other type of control, so it's important to know which games you want to focus on.
I took the easy way out and used a pc wheel without modifying it.
If I had it to do over again, I'd probably use original arcade controls and hack a pc wheel for the interface like thesharkfactor did in the link above.
Try the games out in MAME before you build anything. Most people end up with far fewer games than they expected when it comes to driving cabs.
Most 3D driving games aren't properly emulated yet and the few that are require a very fast computer to run on.
A link to my driving cab in my signature. It was built with Sega Model 2 driving games in mind (Daytona USA, Sega Rally, Etc). Those games aren't playable in MAME, but are with Nebula's Model 2 emulator.