under a 67 convertible which i assume is the same as the other wheel wells, you have 11.5inch inbetween the inner and outer wheel wells, if you roll your gaurd and put in stiff suspension you have abit more to play with 13.5/8inch from inner to outter lip.
I have the same dilemma atm with 20x9 (remember that the US measure from the inner bead to inner bead so a 20x9 is really 20x10 from outer to outer of the rim) I reckon I will need a 6inch backspace though.
here is the meaurements and diagram I made up of the back, i hope it helps