There are a lot of options for rims and tires when it comes to 99-04 Mustangs. But there are a few things to consider when making your decision...
A staggered setup will ADD understeer to a car that came from the factory with a tendency to understeer. You won't add much understeer going with a 265 F/275 R setup, but there's no need to run two different sizes that close to each other. You can run a 275 square setup on these cars. On my 2004 V6 (with an IRS), I've used 18x9" rims with 275/35 tires, 18x9.5" rims with 275/35 tires, and 18x10" rims with 315/30 tires on all four corners. I used the 18x9" and 18x9.5" rims with 275/35 tires on my 2000 GT when it still had a solid rear axle.
If this is for a performance driving application, you don't want to run a staggered setup on the car.
It's also important to note that no two 99-04 Mustangs are exactly the same. Production tolerances weren't very tight when these cars were built. You might have a different amount of "poke" from one side to the other up front. Wider tires can rub in different places. If your car is really low, you might have to roll the fender lips to get some tires to fit. You will probably also need to use steering rack limiters to keep the tires from rubbing against the sway bar at full steering lock. Depending on the mods you've made, you might run into fitment issues with some larger tires.
I'm going to suggest some rim and tire options, but keep in mind that they might not all fit on your specific car.
Generally, you can fit an 18x9" +24mm offset 5.9" BS or 18x9.5" +20mm offset with 6.04" BS with 275/35 tires. Some people can get the 18x10" +20mm offset 6.3" BS to fit with 275/35 tires; fewer people will be able to get an 18x10" with 315/30 tires to fit without modifying the front fenders or using spacers.
I've used multiple SVE rims from LMR (2000 Cobra R, Drfit, S350) and they've held up fine for autocross. Some of their wheels are pretty heavy.
Remember, a 275/35 tire from one brand can run wider or narrower than a 275/35 tire from another brand. If you're trying to fit the biggest tires possible on your car, it's wise to look at the tire specs before you buy.
There's helpful information about choosing tires in this video (some of it is autocross specific):
If you want to see what I had to do to put 315s up front on my 2004 V6, check out this video: