Problem you are fighting is not the tire size but the fact that you are trying to fit 8" wide wheels with only 3-3/8" backspace. You need a 7" wide wheel at that backspace, or 4-1/4 to 4-1/2" backspace with the 8" wheel. The increased width with small backspace makes the wheel sit out farther from the spindle centerline between the ball joints, making the outside edge of the tire swing a bigger arc when turned. This is why it is fouling the SS skirting.
A 60 series tire may get the job done, but I can't say for sure from a distance.
The 8" rim with 3-3/8" backspace also won't fit real well on the rear without jacking the car up to keep the fender lip off the tire sidewall.
All G-body's from 78-82 have 14" wheels, after 1982 it was 15" on performance models like Monte SS and Buick Grand National and Hurst Olds/442. Most standard low performance models after 1982 still had 14" wheels. It's just the way GM did it, don't know why.