I believe so but the axle flange will be a half inch further out each wheel so watch the tire fitment.
so you say is half inch wider on each side, you know how long is the total? (from flange to flange) i have to day a 70 olsmobil 442 rear ,
Flange to flange of the housing 55.25 for the 1968 54.25 for the 1965 rear. OVER all drun to drum where the wheel bolts up 1968 is 61 ish and the 1965 is 60" like a camaro. The gentleman that posted before yours was correct. Build the narrow rear there are more parts available as it is original to your 1965 car. Is there something special about the 1968 Buick rear? Posi gears etc? Jim