[bret]

An 18x10 wheel with a 5.75" backspace and a 275-35/18 tire will fit with [as our website and catalog indicates] a minimal amount of inner/outer fender modification on the 67 and 68 Camaro. The 69 is similar but I have not personally sttod next to one to bless the fitment yet, although we've had several customers report good fitment.
The definition of "minimal" is subjective, but my definition is modifications that can be done by most hotrodders with average experience, with commonly available hand tools in an evening . I understand that still may not clear it up but I don't know how to define it further.
Having said all that, we installed a set of 18x9.5 wheels with that same 275 tire on our 68 Camaro and didn't trim anything all all. No clearance issues except if you drop it in a hole while turned full lock.

The other variable here is ride height. I don't have the fender/ground measurement in front of me [but I'll get it], but it could be that you have selected a ride height that is lower than what we selected. No shame in that at all, but it will change the tire clearance and must be accommodated.

Also...we can't account for any variation of component combination someone might be running, no matter how good it sounds on paper. If you substitute control arms, spindles, steering arms...the tire clearance might be changed...just no way to verify all the various combinations.