245/60 is a better choice, especially since the '74 is like all the rubber
bumper cars, higher on the frame.  The 235 is a bit small in the wheelwell.
The 255/60-15 is run by many as it is much more common and easier to find,
but depending on the car, can rub various spots like the front inner fenders
and the emergency brake cable bracket on the trailing arm.  In '78 when the
255 became an optional tire, Chevrolet trimmed the front half  of the upper
portion of  the front fender lip.

If you run the 255, you need to mount one and trial fit it, checking
everything at all four corners.  The 245 and the 235 can both rub the frame
on full lock turns, but almost all tires will. It is one reason they used
carriage bolts to mount the idler arm and steering box.