Buick 8.5" rears were not installed until 1971 and were the same width as Chevelle rears. Now, if it is a 64-67 Buick 8.2 rear then it is narrower by about 5/8ths inch on each side. If it is the 8.5 rear then the 15x10 w/5.5" bs will work but if it is one of the earlier 64-67 8.2 rears then 5" bs would be about right.