langan, Stock track for a Mustang II is 58inches. My kit is VW based , much smaller than your kit. I am building it as a Ford 2.3 front engine. I cut wheel/tire diameters from plywood and set them under the fenders to establish the wheel mounting surface then with wheel tire combinations determined the track width which is 48 inches for my build. The mock up in the photo is using the A arms from my ’39 build.
The rear will be Ford 8 inch narrowed.
Mel