I've looked at the numbers, and the S197 stock 18 x 18 1/2 has a 50 mm off set with a 6.82 backspacing. So if I were to go with a 2" fill on the backside I should get a 8.82 backspace with the 50 mm stock off set. I would def double and tripple check before I had anything done, that's one of the reaasons that I was looking for input and if anyone has done this with the stockers.
Why do you say that widening would be more than new wheels ?
Your numbers are off here, :sorry
I see you meant to write 18"x8.5 (not 18.5"), but +50mm is 7.22" back-space, not 6.8". I would not go much farther than 7 3/8" max, without figuring to do some hammer work under your fenders or get rubbing. I know a 10.5" wheel will fit, but not sure of what the off-set needs to be. One can run spacers, but think a 1" spacer is too much to run safely, but others disagree with me.
By the time you ship wheels to/from Weldcraft and get them widened, this is about the same cost as your average aftermarket wheel purchased and shipped to you.