4rd Tough,
I always recommend going as long as possible on the tires that are already on your car. Stepping up to bigger/more expensive tires when you’re just starting out has downsides…
1. The best tires cover up a lot of mistakes Novices make, and can lead to bad habits forming.
2. You will chew up good tires much faster if you overdrive the car, and they cost a lot of money.
Driving around the limits of your car and your tires will make you a better autocrosser. I used NT555s (300TW) for my first season, then stepped up to NT05s (200TW) for a while. I wanted to learn on relatively cheap (and not very good) tires before stepping up to the BFG Rival S tires (which are very expensive and very good). I went with Rivals when I got within 2 seconds of the fast local guys in CAM.
One of my good friends runs the 275/40/17 Falken Azenis RT615K+ on his 17x9 rims. He’s very happy with them (and very fast). I prefer less sidewall, so I use 18 inch rims.
Pinching the tire .5 inches off the measuring rim width isn’t that bad. A lot of people do it. I ran 275/35/18 Rivals on 18x9 inch rims with no issues. The most common size aftermarket rim width for these cars is 9 inches, so a 275 on a 9 is widely used. It’s not ideal, but I know a lot of fast Mustang drivers that use that combination without issues.
What will hurt your performance is a staggered setup. Staggered wheels and tires add understeer. Understeer is not good for an autocross car. A staggered setup negates other suspension mods you make to get the car neutral. And, you can’t rotate a staggered setup. You will have to rotate your tires much more often if you continue to autocross. I get 3,000 miles out of a set autocross tires doing everything I can to manage the wear on the outer edges. Your front tire edges take a beating; they will wear out fast if you can’t rotate your tires.
In my opinion (and based on my personal experience), the best hassle-free setup for a 1994-2004 Mustang is to run 18x9.5 inch rims with 275/35/18 tires. This allows for many more tire options. There aren’t a ton of rims available in this size. I used the SVE 2000 Cobra R wheels (+20 offset). They weigh 28 lbs. each (which is a lot), but they are relatively inexpensive. Miro makes a set of 18x9.5 inch rims; they are more expensive, but lighter. I know someone who uses them and he’s very happy with the quality. That +20 offset is the key for fitting 18x9.5s on our cars.
Trying to fit 18x10s with 315s on a 1994 Mustang will require some cutting and flaring of the fenders (front). You MIGHT be able to do an 18x10 with a 275, but that combination doesn't fit on every car.
Honestly, if you know your front rims are OEM 17x8 Bullitts, you could get 2 more for the rear and run a 245/40/17. That gets you less sidewall than stock, more steering feel, and the money you save on tires can be better spent on other mods or more events. Do a season on those tires and upgrade from there. But, some Bullitt wheels don’t fit over bigger brakes.
As for the brakes…
The OEM 94-04 brakes are not great. 99-04 cars have better 2-piston calipers, but even those get overmatched under heavy braking.
I have used 13” OEM 94-04 Ford Cobra brakes with Hawk HP-Plus pads on all 3 of my cars. I am frequently entering tight elements at the rev-limiter, which is about 65 mph. The brakes do everything I ask them to do: great initial bite and no fade. 14” brakes cost a TON of money and they are more suited for track applications. Bigger brakes don’t actually stop the car better, they just dissipate heat better. Well, if you use your brakes as little as possible on course, they won’t fade from getting too hot. Why pay a bunch of money for something you are trying to use less and less every time you autocross?
A really good set of pads and SS lines will make a huge difference, even on stock brakes.
Contact MPS Auto Salvage in Georgia; they rebuild OEM 94-04 Cobra brakes. I’ve used them for years without any issues.