I been kicking the tires on this same issue. I currently have TTII's 17x7 on my 1966 FB. I do have tire rub under heavy throttle in the rear and some in front when hitting a pothole. Front Tires are 215/40/17 and rears are 235/45/17. (26.5 OD)
I'll be changing to shelby 10 spoke, 15x7, probably with BFG T/A's that measure 215/60/15 front and 225/60/15 rear and just shy of 26 OD.
But my 66 has less room than your 68 in the wheel well. Good Luck!
1966 "C" Code Fastback, Ford crate 306, T5, 3.80 Trak Lok Rear, Sub-frame connectors, Tri-Y headers, Shelby drop with an empty wallet.