(quoted from post at 18:21:51 08/09/22) I just worked on a K161 that has 1/2 pipe thread drain plugs in it. I would think a K321 would use 1/2 pipe thread holes in their sumps too.
P.S. If you want to avoid a lot of pain in the future, put a steel plug and/or reducer in there. Brass tends to weld itself to the cast. The only drain on one of those that I couldn't get out was a brass plug. I almost never have problems with steel ones. If you are not worried about original appearance, there should be hex plugs available that you can remove with a regular socket.
If you go with a square plug, there are 8-point sockets made that fit those square plugs really well.
Sorry no pictures. I needed to mow stuff yesterday.
The square head 3/8 plug was not coming off with a 3/8 open end wrench it was rounding off, it's close to 1/2" thread. It won't drop in a 7/16 socket, almost fits a 1/2, and sloppy in a 9/16. My guess is some sort of 1/2" fine thread, not tapered like NPT
The reducer thread is close to 5/8 thread, no go in 9/16, loose in a 11/16 with 11/16" hex head. Again fine and don't look like a NPT.
I could get an 11/16 6 point socket on the reducer but was an interference fit up against the part of crankcase pan that hangs down; I had to tap it on with a hammer then used a 3/8 breaker bar. Had to repeat this several times as it wanted to twist and pop off. After a few turns it spun right off.
I clamped the reducer in my vice on the hex, then put a short 3/8" extension on the square head of the plug. I used a wrench on the square head of the extension (where a socket would fit) and was able to separate the plug. All the threads looked good.
At this point the dirty oil is fully drained and it's getting late. I decided to reinstall the plug assy using the 3/8 extension. I could get it good and snug and the square extension had good purchase on the 3/8 head of the plug. I didn't go gorilla on tightening it down.
Next oil change I'll use the 3/8 extension to remove the plug. Usually the adaprter comes out with it , and some pictures. That should be before end of fall.
Will get some pictures and return to this thread.
BTW, the Ford/K321 is 50 years old, and nothing has been replaced inside the engine yet, just tuneup parts, air filters and carb rebuild from when I used E10 for a while. Smokes a little but still good power.
I ran it for a few hours with fresh oil and all was good.