I re-read through the thread looking for what you are going to do about the gas pedal and I didn't see anything, but you know the gas pedal is a pot, right? I don't think you'll be happy with a microswitched driver. I built one and found there were just so many variables. I used a microswitched set of pedals and found that it's 100% gas or none - lots of the good racing games are not playable like that. You can't depress your gas pedal down 75% and drive at a slower speed. You were always gunning it when giving it gas. Some games like Outrun and Pole Position just weren't playable. Long story short, unless there is some fix I'm aware of, the buttons on the CP for the gas won't work all that well. I even tried pumping the gas to give it gas sometimes, but the games didn't like that all that well either.
The shifting is tough to because mame wants to use 1 button for shifting. Shifting from Hi to Lo back to high didn't matter, it was the same button. I used a return to center shifter but it still was confusing to know which gear you were in. Perhaps there is a solution for it now, but back when I built mine there were a few games which would display which gear you were in. Either way, the problem with shifting in Mame was there wasn't a dedicated hi and low button that gets depressed.
The steering wheel on some games is 270 and 360 on others. Outrun is a 270 game, but I didn't have much issues with a 360 steering wheel.
I apologize if you covered these issues earlier, but I read through your first few posts and didn't see how you were going to address these issues. My game only accurately played a couple games and I would have needed to add a pot gas pedal to make it playable with Outrun.