Appropriate Wheel Offset for 18" Wheels?

I'm curious now... Because I know there are quite a few cars with a 275 series tires and I never here them mention needing offset shackles. I've got to do some research. I autocrossed with Tomswheels and his '67 Valiant and he was running 265/35/18's all the way around (granted he had an 8.8 in it). What about the Red Valiant or the Green Brick? Anybody know about those?

What needs to be modified for the 1/2" shackles because the idea of how they work isn't really connecting in my head. I'm assuming the perches need to be moved in and the offset shackles move them on the frame side, correct? If not, it makes more sense to pay $150 for the kit instead of $250 on tires (just back two). If so, I'll just do a mini-tub years down the line and move the perches then.

Also, the car needs to be on all 4 wheels within the next 2-3 weeks or so (before school starts hence the rush).

Tire profiles vary quite a bit too. Your friend's Valiant fitted with 265s may have gotten some tires with a slightly narrower tread width, i.e., he may have gotten lucky. I know that 265 BFGs would not fit on my car at it's current ride height, but 255s do.
Herrens old Valiant ran 17x9 Cobra rims, tire size unknown, and near as I can remember, the green brick runs 14" wheels, or did last time I read anything about it.

"There's also that weird hump at the top that Darts that could be an issue in full compression." If this refers to the inner fender, mine received the business end of a big ball peen hammer for extra clearance. Crude but effective.