It's been years since I've looked for 13" tires, but it was realitively easy because all the Spec Racer Fords run 13" wheels. Back then they ran Yoko's 008s and BFG's R1's at one time as well. I have no idea what the current spec tires are.
Now, anyone have 245-50-16 take-offs near Milwaukee, Wisconsin (yes- I konw Berget is 1 hour away).

Ian