So what's the customer budget? Are we only trying to get it running? Or does the customer want it to run right ...
You could just put SAE 30 HD from Tractor Supply in there.
If it were mine or I was going to keep it running for somebody, I'd add a canister filter (they were optional on some early Chrysler products) and then run Delo 400 15W-30 SD (severe duty). The filter can be found on ebay or maybe just ordered through your local Napa store...
The engine does not need that oil per se, but the add pack is stout, and it's a very good oil with a narrow viscosity spread.
So have you had the tappet covers off? That is where you will see grunge if it's there.
I'll bet it needs a valve adjustment if it's been running unleaded... The exhaust valves may have receded some, tightening them up and loosing compression ... For the adjustment, I'd run them on the loose side of spec.
If the tappet boxes are pretty clean, it'll run multi-grade high detergent oil just fine
Any HDEO 15W-40 would be very good