I don't have any experience with spacers. Maybe I'd use your wheels with spacers for the street and use the OEM wheels on the track.
Do you think putting 305s on the rear would eliminate the need for spacers? Then I'd be in business because I'd want to put different tires on anyway (soon, not immediately).
As you may surmise from my username, I'm out in Lakeway. What part of Austin are you in?
I think I'd like to proceed with this if we can make the schedules work.