275/40r18 will fit, but........

Why would you want such a wide tire in front?? That car is gonna dart all over uneven road surfaces, besides Mustangs handle better with a staggared tire sizes.

That's not really true, you can have the same sizes or staggered so long have you have the springs and swaybars setup right. Give the stock setup is designed with a certain amount of understeer, staggering with larger rear tires should actually increase understeer. Now, aftermarket springs and swaybars will be different.
Dan
 
I had 275s all around on my previous S197, and yes, the car will wander on extremely uneven and crappy roads. (I live in SOCAL, it's full of them)

However with the suspension setup I had on it did make the car handle very neutral. Which was impressive for such a nose heavy car. Equal width tires all around was part of that equation. Since as noted the car already understeers.

Course now with my current car I ain't touching anything on it. The Ford Racing kit with the stock 235/50s handles amazingly.
 
I run 275s all around and it's just fine, even in the rain. The tire you choose to run may also play into wandering. I use BFG KDWs and they are fine. I've run narrower tires on other cars that followed grooves in the road worst. The original Pirelli P-Zeros were the worst I've ever experienced.
 
KD, KDWS, RE050s, RE070s, all are great summer road tires. None are that great for drag racing either street or strip but offer awesome road holding and steering response for all else.
 
I didn't have worse wandering problems with Pirelli Pzero Neros (GT take-off 235-55-17) than with: Bridgestone RE-01Rs at 245-45-18 were twitchy under light braking at low speeds; 275-35-19 Michelin Pilot Sport PS2s are a little twitchy in low-speed, light braking; no rubbing or other major problems. All on a V6 with Steeda Sport springs, Tokico D-Specs, Steeda front bar with bracket braces, H&R rear bar.

Following small grooves in the road ("tramlining"; seems to me it used to be "nibbling") is about the same in all tires, maybe just a bit more on the very-short-sidewall Michelins. That could be a driver-sensitivity function rather than more-gross behavior.

Just FYI, I like the Pirellis for general all-around driving, the Bridgestone for serious curve-making (autocross), and the Michelins ride pretty good for high-speed touring.