widest tire with spring relocation

I don't know what your car measures...

But I have my layout notes on my '73 Duster that could be used to back figure other models. I used the mini tub & spring relocation kit from US Car Tool.
(All dimension where taken from vertical plane through rear axle on a layout table).
Outside of rear fender to outside of rear fender = 71 5/8"
Inside of fender lip to inside of fender lip = 68 5/8"
Inside of trimmed fender lip to trimmed fender lip = 69 5/8" (1/2" each off lip)
Outside of frame rail to outside of frame rail = 38 7/8"
After mini tub I have 39 1/4" clearance between tubs. (It's just the way it works out when you rotate a tire in the mini tubbed fender well).
I got 15 3/8" of clearance +/- 1/8" (They aren't perfectly even side to side do to production tolerances).

What I did get was enough room to properly fit a 355/30/19 tire with room for it to move while cornering.