Yea, the P-255's were optional on '78's. The car I bought back in '78 was a L-82 4 Speed FE-7 car with the P-255's tires on 8" ralleys. On the front, the fender well lip was factory cut back flush with the side of the inner fender. The lip was cut back for tire clearance, as the FE-7 cars sat a bit lower.The rear wheel wells were un-modified. The question I would have is how much did the body change in the wheel well area from 1977 model to the 1978 model. I have the feeling the answer is very little. If you could find a fellow Corvette owner local to you with P-255's on his car, perhaps you ask to borrow a wheel for trial fitment. Or talk with your tire dealer, he might mount a tire up for a temporary fitment. I have done that in the past with my local Discount Tire dealer. If there is a problem, I would expect it to be on the front. The emergency brake cable bracket shouldn't be a problem with the P-255-60.
Side note - my '78 was ordered with the YJ8 aluminum, but was built with the steel wheels, due to porosity problems with Kelsey Hayes manufactoring the wheels down in Mexico. Dealer assured me that it wasn't a big problem, take the car with the steel ralleys, and the Dealer's Parts Department would order a set of YJ8's and install them. Got the car, Parts Department tried to order a set only to find out that GM required a VIN number for a car with the factory installed YJ8's. Finally got the wheels, using a VIN from a Silver Anniversary sold by the dealer.
FWIW,