I've researched this issue extensively as I wanted to re-stripe my Shelby GT in blue and add a set of vintage Torq Thrust wheels to give it a throwback 1966 GT350 look.
The only "standard wheel" option is to go to 17x8 and 25mm offset (pn: VN2157867). Otherwise, you are going to pay $500+ per wheel for a custom wheel order for 18" wheels that are 8.5-9" front and 10-10.5" rears. Personally, I'm fine going with 17's to add a bit of sidewall height similar to how '66 GT350's were setup) but it does limit you to a 235 width 17" tire (factory sizing) as there aren't a lot of good 17" tire options out there.
It's ridiculous that American Racing does not make any standard wheels (one piece) for the S197's with the proper 35mm+ offset while allowing a nice tire width combination.