The 205/40 R17 is the same width as the Cosworth uses, but a slightly bigger diameter. But it is actually the same diameter as for a standard Sierra, which uses 185/65 R14. Most people (including me) run 215/40 R17 on a Cosworth, which is slightly wider and also bigger in diameter. Next to that my wheels are 8J wide and ET35. I assume yours are less wide, which will help with the 40 mm offset you have. Standard Cosworth wheels actually slightly catch the strut on a standard Sierra. This is because the strut/knuckle on a standard Sierra gives less room for bigger wheels.