275/35r18 will fit easily. The wheels I have now are 18x9 and the tires are 255/40r18. They fit with and without a 7mm spacer. This picture is with the 7mm spacer. Still plenty of clearance on the outside. For a 275/35 the ideal wheel is 18x9.5 which is 12.7mm wider, the same offset would put the tire about 10mm wider to the outside; the 3mm difference is less than 1/8". The 255/40r18 is a taller tire. I chose the 255/40r18 and the 18x9 because the 18x9 was $140 and the 255/40 is 26" tall. I used this wheel and tire for fitment at the body shop and checked clearance with and without the spacer; this way I know the wider tire and wheel will fit without issue. The wheel I want to change to after I start driving is much more expensive. Keep in mind I rolled the lip on the inside to buy some margin and also modified the fender in front of the wheel for tire clearance.
More on that here:
1968 Barracuda Front Fender Modification for 26" Tires
The rear has relocated springs and mini-tubs. I currently have 295/40r18 on the car with over 1" on the outside and over 2-1/2" clearance on the inside. I can fit a 345/30r19 back there, but I may run a 325/30r19, have not yet decided.
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