I had the same problem with my ERST, ERST wheels will NOT fit if you have 2wd 4 pots put up front. You can either have the front of the calipers machined down or use a et35 rim ie a cossie lattis. I would suggest getting a steel rule out and measuring exactly what the off-set is.

Dont forget if you have done a rear brake conversion, the rear disc kicks the back wheel into the arch buy about 5mm.