The big block chevy heads have a staggered exhaust flange bolt pattern as well as the 6.2/5
I measured both sets of my bbc heads here by my computer desk a set of "840's" and a set of vortec's... both (which there shouldn't be a difference), but I measured both anyways becuase of the exhaust port size difference, have a 4.875" or 4-7/8" spacing.
My old 6.5 heads measured 4.75" or 4-3/4" spacing. All spacing measurements are from center to center of the exhaust ports. From what I just came up with, I would think if a nice "heavy gauge" flange was lazered out for the 6.2/5.... A person could probably cut the flange off from a bbc header and I'm sure there is enough "massage" room there to make them work. Would definitely be very interesting to see and or hear the results.
Also did forget to mention, that the bbc headers are a round tube where as the exhaust ports of the 6.2/5 are a tall narrow rectangular shape, for those whom may not have known.
Mike