Motor's got to come out and be disassembled by a qualified mechanic or engine machinist to see how much damage you actually have.
Sounds like you're describing connecting rod knocking, which means your motor has the equivalent of congestive heart failure for a person.
You might do a compression/leak down test to see how much damage it did to the cylinders, piston rings, and heads just to be sure.
Some of the experts may want you to check a couple of things before it's pulled.
You're probably in for a full engine rebuild or swap it out with a re-manufactured 3.0L 4 cylinder ready for install. Might have to go up a size in jets on the carb, would be worth getting the experts opinion here.
Oil drain plug coming loose? It's hard to see that happening if the people removing and installing it knew what they were doing. Not making any assumptions here on who is to blame, just doesn't add up if it was put in correctly without being stripped.
Stripping the threads and leaving it too loose are probably the two most common mistakes I can think of.
