The daughters truck has 18's with stock spindles, no inner fenders (front/back) and will lay frame without touching the clutch master or brake booster. 8" wide rim if I remember right.
As for the 8.5" rim it all depends on back spacing. The rims on the daughters truck probably should NOT have fit but.... where there is a will...
Anyway, if the fit is close there is ways around the problem.