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.
