I was doing a search for something else regarding the 8 3/4 diffs and found this thread. Actually Rob, those numbers are dead on. They are not talking about the flange to flange measurement on that site. They specifically say "track width". Track width is the distance as measured from the center to center of the right and left tire, therefore, those numbers are correct. They were just not what the OP was looking for.
You might want to look at what is written on the site, it says Flange to Flange.
Copy and paste from the site referenced.
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B - BODY 8 ¾ SWAP INTO A BODY
I keep hearing that B body 8 ¾ rear ends fit into A bodies except that they are a little wider. I have never seen this swap but I wanted to provide the B Body 8 ¾ dimensions verses the stock A body 8 ¾ setup. The first measurement is the centre to centre spring pad width of the housing,
the second is the total axle width flange to flange.
67 - 72 A body 43.02" 55.60"
62 - 70 B body 44.00" 59.20"
71 - 74 B body 47.30" 62.00"
71 - 74 B body Station Wagon 47.3" 63.4"
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The rear track on a 66 Coronet is 58.5, not 59.2