the B body diff should be cheaper than the E body , the 66 -70 B body is slightly narrower as well , there are only 5 Mopar diffs , the 7 1/4, 8 1/4, 8 3/4 , 9 1/4 & the 9 3/4 dana , the A body only used the 8 3/4 in the 4 " bolt pattern
the 2 strongest diffs are the 8 3/4 & Dana