Hi Liv4spd,
You are going to get your best advice directly from SPF, I wouldn't hesitate to call them if you haven't already, or from other SPF owners who have 17" wheels. I have an ERA and have ordered 17" pin drive wheels from PS Engineering, which I'm still waiting for by the way. I am using a 17"x9" wheel with a 275/40ZR17 tire in front with a 5.30" back spacing. In the rear I am using a 17"x11" wheel with a 315/35ZR17 tire with a 4.50" back spacing. I doubt if these numbers will be correct for your car, but they should be in the ball park. I would also call Trigo or PS Engineering (I will mention they are hard to get ahold of, so keep trying, and try early in the morning like 6-7am PDT). They have undoubtedly made 17" wheels for SPF's before. It would be best to confirm the numbers from at least two different sources, since that is not something you want to get wrong. Measure twice and cut once.
Hope that helps some.