You're quite restricted in choice on 18 inch wheels. However, 265/60/18 will fit ok. Yours are presumably not standard wheels - some did come with 18 inch, but not in a 8.5 inch width. However the 18s that mitsubishi make have 265/60/18 tyres. That's the same overall diameter as the more common 17 inch wheels with 245/65/17 tyres. The truck will take bigger - up to 265/70/17 without mods, but I cannot find an equivalent size in 18s.
Thanks for the info I will go with the 265 60 18 cheers benshepherd