I am a bit confused

Nothing new there but I think some of you may be able to help.
I read in a post somewhere on here that you had to use 15 inch wheels as the standard Sierra wheels foul the rear upright assembly. This seems a bit strange as there does not seem to be any mention of this in the book.
Can anyone state categorically that the standard Sierra alloys will fit?
I have just bought a Sierra with an option on a second set of wheels but don't want to buy em if they wont fit.
Any inoput very much appreciated.
Enoch.