The magic numbers in the front are 8.5+30, 9+35, and in the rear it's 9.5+35, 10+40, 10.5+45, 11+50. Be careful about going wider than 245 (255 should be fine) in the front, and 285 in the back (especially with that 11+50). 295 will fit but will be very close to the suspension, but if you drop to 10.5+40 (you're probably not going to find this unless you order custom) you can probably fit a 295 and maybe a 305 but it's getting really close, YMMV, but those magic numbers above have been tested just that that 11+50 will be snug, just go with a 10.5+45, there are tons of off the shelf options because that's a Mustang fitment.
You'll get a little poke if you go down to +20, which might be alright if that's what you're going for but I don't think we have the right shape of fender for that to look good.