I ran 9.75's up front and I had a small amount of clearance but that was with 255's. You will need a wheel with the right offset and bs. Most 18x10 "mustang" wheels will require a spacer. I remember seeing a black sn95 cobra with black 18x10 fr500's all around. IIRC it is the 98 drift cobra
I don't know how great that will be on the street. I had issues with 275's jumping in and out of the ruts the big trucks made in the highway and dropped back down to a 255 on a 9" front. I would assume you will be running at least a 275 on a 10" front if you go that route.
Shifty Powers is running a 295 in the front. With a tad bit of camber you could easily get away with 305s still in the fender. The tricky part is getting the right backspacing on the wheel so that you don't require spacers and the wheel doesn't rub on the inside.
As far as offset 3/8s is an additional -9.5mm on a "normal" mustang wheel ......I need to wheel shop I'm just gonna do it and deal with the outcome lol