You're looking at about 4 7/8 backspacing max for 17s, I believe Elcos have a little more room in the back but I'm not 100%. Never drove it, but I had a 17x9.5 with 275/40/17 on the '78, seemed to fit ok, but they might rub on bumps, uneven drives, etc.
Backspacing is the wheel mounting surface...