Will 235/40/18's fit?

That size of tire is really too wide to go on the stock MS6 rim. Now if you go wider rim you should be ok but it'll depend on the rims offset as to whether you rub or not.
 
225' 40's aren't even recommended. Although lots of ms6 guys are doing it with no issies, but on a 7" trim, 215 is the max "safest bet".
 
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