Oil in a 1981 Pantera

if you havent figured it out yet the drain should be a bolt inside the tunnel and any chain case oil will work just ask a dealer how much and they should know

I can look on my 1980 Trail Cat, should be similar. I just did the oil last yr and recall a plug near the bottom, ie fill it up to that plug, couple inches from the bottom.
If you go to the alpha-sports website, they have the parts diagrams all the way back to the 70's models.
Chaincase is the same p/n as my 1980 TC and I remember a fill plug near the bottom. Fill it to that and you'll be good.
I don't recall a drain plug, but you should pull the cover (4 bolts) anyway to inspect the chain, gears and tensioner and claan it up. Be careful with the rubber seal to re-use it. Put a light coat of rtv sealant on the gasket if it's not in perfect condition, at least around the bottom few inches where the oil sits.
On older chaincases, about any oil will work. Do not use gear oil with high viscosity due to low oper temps. Chain case oil reccomended, but engine oil, manual trans oil and even atf will work.