I am running a 255/45R on 17x8's on my 65 with a 5.5" offset and 3/4" Mopar spring perch relocation setup. I have room to probably get away with 275's, but it may rub when the rear articulates turning into driveways.
](*,)I just realized that if you're running the drum brakes, this offset will be of little value to you, as I am also running a disc setup that kicked the flange surfaces out, I believe, 3/8" per side when combined with Moser axles.
In any case, the 255's fit with no problems. Sorry, that probably didn't help much.