Good question. I’d have thought a 9”, but a search on the HAMB turned this up. Based on the weight, I’d go look up the Dana 53. The 44’s a good unit, but I don’t know it was used on that heavy a car. My Dad had a 62 and a 67, and they were damn near 7000lbs optioned out. Felt like it in a corner or on the brakes.
Lincoln cars used that axle for a long time. It was the top strength Spicer Dana unit for cars but they are not easy to source parts for now days since production was limited. Soft parts are likely still available but hard parts not so much. They are hell for stout though so used units would likely still be in decent shape as long as they were relatively well stored.