We did a number of things to address drivability:
We added a flat plate to the left of the clutch pedal to rest the driver's left foot on. This is convenient when driving on the street; it's a lot more comfortable to have a place to rest your foot than to just have it dangling.
On the track, it's an essential. You can use the dead pedal to push yourself back into the seat, helping to hold your body firm as you are subjected to the high lateral forces going through corners. This allows the driver to be much more precise with steering and throttle inputs.
The stock FFR setup has no up stop, and if you pull the brake pedal back far enough, the pushrod falls out of the master cylinder, leaving you with no brakes. FFR suggests that you attach a spring to the pedal to keep it from coming back too far, but I didn't like this approach. It doesn't seem positive enough to me.
I designed and built a simple adjustable up-stop that works great. It's a piece of angle aluminum bolted across the square tubes which support the pedal box, threaded for a bolt and stop nut which contact the brake pedal shaft a few inches above the pedal. The bolt can be adjusted to make the pedal stop at just the right point.
The stock FFR clutch pedal setup results in the clutch pedal having far too much travel, making it difficult to find the point where the clutch "bites", and also causing you to move the pedal way too far to the floor every time you disengage the clutch.
Nate added a down-stop to the clutch pedal which limits its downward travel to a range which is adequate to properly disengage the clutch, but no further. I didn't think this was essential but after trying Nate's tweak, I agree that it is a great idea and I wouldn't want to be without it.
The stock Mustang pedal in the FFR installation results in a throttle with extremely nonlinear travel; you get about 75% of the power in the first 25% of travel. It's extremely difficult to get the car rolling smoothly, especially in first gear; it tends to lurch as soon as you tip in the throttle, then your foot comes off the throttle a bit and the engine compression causes you to lurch forward, causing you to put in the throttle again, causing it to lurch back again, etc. It's also difficult to modulate the throttle coming off of slow corners, something which is especially important in autocrossing and racing.
Nate designed and built an extremely clever modification to the pedal which causes the ratio to be much slower in the first part of the throttle travel, getting progressively faster as the throttle opens more. This gives much more linear power progression relative to throttle position and makes the car much easier and more fun to drive on the street and also makes it much easier to modulate the throttle coming off of slow corners.
Go here to see some photos and more details.
After struggling to stay in the seat for our first few events, we bought a Cobra Clubman Sprint seat from Sube Sports. This seat was cheap, and works great.
The only difficulty was fitting it into the car, because of the 4" frame rail under the right side of the floor and the fact that the seat must be bolted in from the bottom. We had to discard the seat track, and Nate made a bunch of brackets to bolt the seat to the various tubes under the floor. It just barely fit, and the right front bolt just barely squeaked past the 4" frame rail after a lot of careful fitting.
When we drove the car with this seat at Lime Rock, we found it made a dramatic difference. It was much, much easier to steer the car because we didn't have to clench the wheel in a death grip just to stay put in the seat.
Update: Later we bought another Clubman seat from Sube for the passenger side. Unfortunately, the excellent Clubman Sprint had been discontinued, and we had to settle for the vastly inferior New Clubman, which caused the passenger to sit hunched over in an uncomfortable position.
Update 2003: After trying a couple of FFR Spec Racers with Kirkey racing seats, I bought a 14" Kirkey Intermediate Road Race seat from IO Port Racing. This is a real racing seat, not a compromise street/track seat like the Clubmans, and it made a huge difference. Read more about it in my May 2003 event report.