Spredsheet Works!
Setting up my '55 gasser, with everything gutted out from under the car, new wheels & tires were set up where I wanted them and mounting surface flange to flange measurements were taken.
Rear: 15x12, 4.5 BS, with 315/60R15 tires, minitub, came out 53" axle width on the rear,
Front: 15x8, 3.75 BS, with 245/60R15 tires, solid front axle, gasser sits 16" high, 58-1/4" axle width puts the outer edge of the tires just inside the front fender lip.
Chevynut's excel spreadsheet confirmed everything exactly, as expected! Everything sits just the way I wanted it to. Now I just have to finish building it!
That's a great spreadsheet, I hope everyone makes good use of it!