Currently I have 255/40-18 in the front on mine with 8mm spacers, there is easily room for the 275/35-18 on mine. The lip is rolled on the front and I modified the leading edge of the front fender. The '67-'69 Barracuda fender has a different shape than the Dart. I made a photo garage on the fender mod:
1968 Barracuda Front Fender Modification for 26" Tires
On the rear I currently have 295/40-18. It will easily fit a 325/30-19 and might even clear a 345; I previously had 325/50-15 Drag Radials under the rear with plenty of clearance. Car has mini-tubs and relocated springs. The fender lips are rolled in the rear also.
Here is a link to pictures of the car undergoing chassis mods and paint:
1968 Barracuda Notchback: Paint and Body 2014
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