Should I be looking for 35 or 28 offset with 13x7

Have you redesigned the rear uprights then?

If making as the book then 14" steel wheels will fit, but no smaller then 15" alloys will fit without fouling on the rear upright.

13's wont fit. i have 15's on mine and they only just clear the rear upright.

Where are they tight fitting? Shock mount or bottom arm I guess

Im running 17's on mine but ive fitted larger brakes up front, 15 inch is the minimum you can run ona roadster due to the rear wishbones sitting "wide" of the rear upright if your using the standard designed uprights.