Stock Rear

Never knew the abody had the 8-3/4 option for 65 and 66. How come the 273 4bbl did not come with them?
When did the Dana 60's come into play for the Big block a-bodies?
S/N: Cosgig, not debating- just curious. I honestly do not know either way. I've never seen an 8-3/4 under an early A-body factory. new one on me.