I also have a FiberFab widebody. I'm currently running 225/35/18 Omni United Radar Dimax R8+ on all four corners a set of Boyds 18 in. x 8 in. Junkyard Dogs. They have a 5 x 4.75 in. bolt pattern and a 4.50 in. Backspace. The previous owner installed them using adapters for the standard VW 4 x 130 mm bolt pattern. When I bought the car, both rears and fronts would rub during not-so-hard cornering. The Empi 9504 adapters the PO installed were 7/8" thick. Changing out the front and rear hub/disks to ones with the Chevy bolt pattern brought the wheels inboard 7/8" all around, which solved my problem. Even in hard, fast corners my tires do not rub the fender wells. The 225 mm. tread width works out to just under 9". You could go wider in the rear by changing the backspace on the rear wheels. I don't recommend it up front, I had to adjust my steering gear stops to prevent rubbing on the insides. I haven't messed with wider tires out back, I like my tires all the same size so I can rotate them on all four corners, instead of just swapping sides. My car sticks through the corners well, with only a 3/4" sway bar up front, none out back on my IRS rear. I have to drive it hard to break the rear loose on asphalt. As far as I know, my suspension height is set up per the FiberFab manual, I've never messed with it. But you be the judge, here's a couple of recent photos.
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