Both the Chevy and BOPC ( Buick, Olds, Pontiac, Caddy ) transmissions were 6 bolt housings for most of their production life. The original Chevy housing was 7 bolt and that's why their trans housings have a peak. In fact, most Chevy blocks were manufactured with the top center bolt hole drilled and tapped long after Chevy had changed to a 6 bolt housing.
So how do you tell the difference between the common as dirt Chevy pattern and the far more desirable ( to us ) BOPC housing? 1. Well, the first and most commonly stated visual reference is the Chevy peak versus the BOPC saddle at the top of the bell housing.Here's a picture of the Chevy ( L ) and BOPC ( R ) pattern bells:
2. Also, only the two bolts below the alignment dowels share the same position between the two patterns. This is why a dual-pattern housing has 10 bolts, all the holes above the dowels must be duplicated. A Chevy bell has two bolts on each side aligned vertically with the respective alignment dowel. A BOPC bell only has the bottom bolts aligned vertically with the dowel and the higher bolts are moved inboard.Here is the close up of the blocks. A Chevy 350 and 1970 Buick 350:
Interestingly, I've seen a factory dual pattern Manual housing within the last month.