I've been trying to find this out too and my best guess is that they were useing 29" and 30" tall tires, 10.50 and 11" wide on anything from an 8" to 10" rim. Slicks for 16" rims were also about at that time, so I wouldn't be suprised if they were being tried as well. Again if anyone can say for sure I'd like to know.
Yeh, I've just had a quick look through those rules and I think the 10.5 tire rule possibly has more to do with not giving certain body styles an advantage over another, rather than historical correctness. Or it might be that 10.50 was the widest tire you could get back then and that rule is there so the cars look "right"?