Good News!!! (and a question)

I'm fairly new to this dangerous addiction of rebuilding old Mopars. Long story short, My '76 dart custom /6 has sat (in various locations) for at least a decade.

I got it running today!!!!!!

Here's the question. Old diffy is shot (or at least I was told so by the mechanic before I parked it for 10 years). Got a "new" one from a valient same size and all.

Should I attempt a rebuild? or just change the oil in the new differential. If oil change, what weight should I use for a 7.25 rear end?