20mm with FEs is not going to work in the front. I'd say 10mm is probably max with 255s. 20mm will be fine in the back though, but with 10 on the front you'll have to use bolt-through anyway so you can pick what you want for the rear. 15mm would put the rear wheels about even with the fronts.
Big Red has bolt-through spacers but I can't remember the sizes. Maybe Dean will.
My new wheels are effective ET 43 in the front and 40 in the back so I have more clearance than before which is good since 20mm all round with the balloon Goodyears was a bit tight. Looked good though
