Several years back, I had Weld Draglites (15 X 8) 4.5" backspace with Mickey Thompson Indy Profile S/S N50-15 (27.5" tall, 11.8" tread width) on the rear of my '66. I also still had the polished wheel opening moulding, at first glance it looked unaltered and everybody that knew better had to poke their head under there. I had ground the lip across the top, stopping just short of the moulding mounting holes. If I had 4 passengers and hit a hard bump while turning they would rub. I would have to say that these were about the biggest tires that a '66 could swallow, 1/2" or so off the outside and I could barely put my finger tips between the inner well and the tire.
With the rearend dropped over the tires, I switched to the L60-15's (28" X 9.5").
Just my experience, hope it helps