Oh and for measuring and verifying fit, I found this sizing link that seems good. If you measure your current tires and put in the clearances, it tells you how much closer you will end up to the wheel well and suspension,etc. Just helps me to visualize.
Post here what you end up picking, I want to do the same thing on my '67 also and it seems like we are thinking about the same style and size. How big can we go in the back?
I have seen some posts on this site with someone using 295/45-18 on rear which is a 28.5" diameter. They are 11.6" wide. On the front he used 275/40-18. Can't remember who that was.
Others have used up to 27.8" rear (275/50-17). My car has the adjustable air shocks on the rear so there is a little room for added tire I think. I plan to get some good measurements before I buy anything. Or take to a good shop that will have some demo wheels to try out.