Hi there nate_torg!
As you're probably aware, settings are an individual thing, no 2 drivers seem to like a car set the same way, so I'll post my settings here, use them as a starting point, but ultimately use whatever feels best for you. No doubt we'll hear from some more of the senior members of GTPlanet saying "Oh no, I wouldn't set my downforce like that" or "Can't believe you do it that way" but that just reinforces what I've just said, each to his own, and all that!
The GT40 is a handfull to begin with, it handles unlike anything else in the game, with the exception of the Zonda (which doesn't steer!) and the XJ220 which is the epitome of Ford Prefect's quote in Restaurant At The End Of The Universe, "...Looks like a fish, moves like a fish, steers like a cow..."
Once you get used to it though, braking massively early for sharp bends and controlled use of the accelerator through the bends, its one of the most rewarding cars to drive in the game.
Here's what I used when I entered my latest GT40 racecar into the Professional league GT All-Stars championship, and saw off competition from the likes of the GT-One racecar, XJ220 racecar, Esperante GT1 racecar.
I put t2 Super-slick tyres on there, a handfull when cold but they last really well, and generally give their best lap times when coming out of the green phase into the yellow phase.
I gave it a wash, oil change and fitted the NA tune stage 1 kit to boost the output to 613HP as I knew it'd struggle without, especially when the oil light came on after 5 or 6 races in the series.
Spring Rate Front 15.7 Rear 16.5 (I find with MR car, it makes sense to have harder springs at the back 'cos they're supporting more of the engine weight, although theoretically the distribution is supposed to be closer to 50:50 in an MR vehicle.)
Ride Height Front & Rear 65mm (Like to keep the car level, but not so low it bottoms out.)
Shock Bound Front & Rear 9 (to go with hardened springs)
Shock Rebound Front & Rear 8
Camber Front 2 Rear 1
Toe Front -0.5 Rear 0.5 (Front toe out slightly to improve turn in, rear toe in to help cornering, not too much of either to avoid tyre wear)
Stabilisers Front & Rear 6. (Like my rides to stay flat through corners. Not to everyones liking though, play around with them)
Brake balance Front 21 Rear 18. (This car needs some serious slowing down, and without the benefit of carbon brakes used on most modern racecars, it pays to brake early in the GT40. More front bias makes it stop better, but will wear the front tyres quicker.)
Limited Slip Differential (another source of endless debate)
I used
Initial Torque Rear 30
Acceleration 30
Deceleration 60 (This helps to decelerate the car with engine braking and I found it slows down better with a high setting like this.)
Gear ratios
I generally just play with the auto setting value, something around 33 or 34 will work for most tracks, though you should reduce it to 31 for Laguna Seca and perhaps lower still for Cote D'Azur. Increase to 41 for Test Course. Bottom line you need to set gears for each individual racetrack.
Downforce.
I tend to use very slightly less front downforce than rear.
Again the level changes from circuit to circuit. I use lowest setting for Test course, and above 3/4 setting for tracks like SS11 and Cote D'azur.
Again bottom line, no setting works equally well for all tracks so you should set it race by race. Higher for twistier circuits, lower for ovals.
ASM and TCS.
Turn 'em off!!!!! I always use 0 and 0 as ASM and TCS settings. Ran my first GT3 game with them on at default levels, 2nd time through I used 3 and 2. Third time I turned them off on every car and every race (even F1s) Now on 4th time through, using Manual for the 1st time. Way quicker, but I now refuse to run with any driver aids turned on. (Except ABS, but there's no option to turn that off! Just in case anyone feels like nit-picking!!!)
Hope this helps.