I just mounted 215/65-15's on 15x6 in rims on my 68 Barracuda. The backspace is 3.75 and the diameter of the tire is 26'. As you can see in the pic, I have about 1' to the front and rear outer fender edge and no interference under any circumstance. I'm guessing that a 27' tire would in some cases rub on either the front or rear outer fender while turning. If you increase the backspace to 4.25 then you would bring the leading edge of the tire further away from the outer fender which may solve the problem of hitting the outer fender, but at the same time, now your tire is closer to the inner fender and you'd be running the risk of rubbing the frame rail or inner fender in a tight turn. Either way, it would be really close.
View attachment 1715225851
