The historic TransAm group does there own thing to some degree. In other series like VSCDA it's a bit different.
I am restoring an old ex-IMSA RS car so I've been looking at what is used commonly, and basically there are good tires available for 15" and 13" rims.
Vintage racing is popular enough now that the low-volume tire makers can support the hobby. Sizes however are somewhat limited, so you need to know this as you are building the car.
Hosier makes good 15's, and many of the small displacement cars at Road America Vintage races this year were on Toyo's. For example the Toyo Proxes RR comes in 2 15" rim sizes, 205/50 and 225/50, but that's it.
I hadn't paid much attention to what the TA guys were using, but they were burning them up like they are going out of style