8" on the rear (depending on tyre width and suspension height) is a good fit, but on the front it may be an issue if you are going to lower it a bit.
I run 14x6" std GTS on the front of my Sandman with 235/60 tyres and a 2" drop in height without any rubbing at all. On the rear I have 8" GTS wheels with 245/60 tyres and it's also 2" lowered. I get a very slight touching of the tyre to the guard over a big bump on the left hand rear only but it's infrequent and not an issue.
On my other van I run 14x7" on the front with same suspension height and no rubbing, but I did run 14x8" with 225/60's and it fouled the guards when on full lock sometimes.... this is where 15" might give you some trouble unless you run a very low profile tyre.
Hope this helps.