Whatever it is, it may have been narrowed, judging by the welds on the ends. It's definitely not a Ford 8 or 9 inch.
Olds/pontiac have angled ribs on the front. 8-3/4 mopar and 49-56 Ford car have round rear cover, not shaped like that. Looks pretty big, too.
Think bigger truck. Ford 5 tons and up used real beefy 9 inch and spicer made some rear axles with a similar kick out in the back plate. What size is the ring gear?
OP, crack that shit open and tell us who's part numbers are on the ring gear, pinion and carrier. Look at the size of the diff vs everything else, especially the size of the axle flanges. That's not a 1-ton let alone medium duty on up truck rear end.