A quick look and I could find no info on those numbers either.
https://www.crankshaftcoalition.com/wiki/Wheelbase,_track_width,_and_differential_measurements
The GM Truck section shows years 73-87 and 73-91 with the second dimensions of 65.8" axle flange to axle flange.
Your rear axle could be from a C10, K10, C20, K20 etc. of those years.
I would try posting (have to register first) at
www.67-72chevytrucks.com to see if they can help with the identification.
They have a section for 1973-87 trucks.
Yes, I saw your post for the car transmission. Looks like you got some good advice on how to make it work.