I thought the 31 splines in those trucks (70-72) were a 9 3/8" limited slip and that replaced the earlier 65-69 5 lug Dana 60's. It was the only way you got a "posi" in a half ton non-4 x 4 before 73.
I have had a couple 3L50 9 3/8" third members, a friend had a 4L11... and it's not from a Drag Pak Lincoln.
But as to the question, i'm not sure those 31 splines will fit the earlier housing. They might have used a different width or pinion offset in them. Hollander manual should show them.