[Fraser]

Wayne,

Firstly welcome to our group. The tire sizes you have are exactly the same setup as I am using on my build (245/45 R17 front, 275/40 R17 rear) and although like you I am still in build this was my thoughts and what I have also found.

Even with 245/45 R17 front I needed to use rack limiters as the inner tire wall rubbed on the F panel at the extent of travel.

With a 26" diameter limit the largest obvious choice is 315/35 R17 on the rear and possible 275/40 R17 front or any combination in between. So my thoughts were to start with the smaller (have already found rubbing) and once complete decide if I want to go bigger on the 2nd set of tires. I guess I won’t be able to resist a few burnouts and doughnuts early on, plus it may take some tweaking to get alignment perfect which could induce excessive wear so don’t envisage keeping the first set of tires to long.

Rear axle can make a difference, 79-93 Fox Mustang width: 59.25” hub to hub, anything newer is wider and subject to rubbing.

Short answer, you should be just fine with what you have.