I can identify with your delema, as my local "friendly Ford dealer" tried to put me in a previously owned GT500 about a year ago...it was tempting, since they offered me close to full window price for my Bullitt against the 98K ask for the GT500, but having to anty up 50K was a tough call for...
I currently have the GT350 wheels, 10.5 & 11 wearing 305 & 315-30x19 Cup2s on FI Bullitt, they fit fine, however with traction control off I can easily overpower the 315s in the lower gears...would include a picture but this app won't let me attach one...
Sorry about the delay in my response, post holiday family stuff took priority over car stuff...would like pictures of curb rash on the rears and would it be acceptable to you to bring them to Arcadia which is about half way to me...
Ok, I just couldn't resist commenting...I have an ESS Kit on my Bullitt I love it and it came to just under $10K shop installed...haven't broken anything yet, but I don't drag race or track the car, just "spirited driving"...just saying.
Back in the '80s, when I lived in California I got by the "censors" with BUT AKE on a Corvette which was truly a PITA...still have the tag hanging in my garage.
My Bullitt hasn't been on track since the ESS install, but as I still have the origional clutch (36k miles) and the flakey MT-82 both intact so I wouldn't worry much unless you do a lot of hard starts with tires that hook up real good as the boost dosen't kick in hard until 3500 or higher and it...
OBTW my local Ford peddler tried to put me in an 8K mile '21 GT500 for my Bullitt and $50K a few months back, that's when I decided to do the ESS thing...in my mind I'm 40K ahead of the game.
I have pretty much the same setup on my Bullitt and don't have any issues with it (so far) and would not hesitate doing it again...this transformed the car into a "poor boys supercar" of sorts, along with a bunch of Steeda suspention parts, and yes you will need bigger tires out back...right now...
I have one of each, a 2 and a 5, I use the 2 on my motorcycle and the 5 on my Bullitt both with quick disconnect pig tails...no more dead or low battery issues....
I made the switch from the 4Ss to the AS4s about 3 years ago when the 4Ss were done at 25K miles, less noise, better ride same level of grip in "normal" driving...but now that I've gone F/I those wheels and tires are stored as they have been replaced with GT350 wheels wearing Cup2s to cope with...