It's highly unlikely that rear quarter panels from a 2-door hardtop Pontiac Bonneville will fit a 1965 Parisienne Custom Sport hardtop. While both are GM cars and from a similar era, Pontiac and Buick (Parisienne's manufacturer) used different body shells and dimensions. Even within the same brand, year-to-year changes can make parts incompatible.
You'd need to find rear quarter panels specifically made for a 1965 Buick Parisienne Custom Sport hardtop. Trying to adapt Pontiac parts would require significant, likely impossible, custom fabrication.
