You're in that really tough spot, Clay. Small, thin, high speed tires went out with the Triumph Spitfire.
Before I bit the bullet and had some 15's made up, I was looking at the Hoosier site. They've got a vintage racing A-70 13 available. Not a cheap tire, not as thin as you'd probably like, but it might work in your application.
Potenza might be a choice. Minimania's description is a little vague, but you could drop them a line.
http://www.minimania.com/web/Item/TIRE21/InvDetail.cfmGood luck with this one - it ain't easy.
Chris