Now I have never had a 641 or even had the pan off any tractors. (Numerous cars, yes)
Idea that maybe good, maybe not.
If we assume its a un-primed oil pump
or bad leaking oil pickup tube.
What if you overfilled the oil to the point
the entire pickup tube was covered.
If still no pressure, you can go ahead and drop pan and "do what you got to do."
BUT on the small chance you do get oil pressure, drain it back down to normal level. Run it, if oil pressure drops back to zero,
IF by a even smaller chance oil pressure holds,
my "guess would be a worn oil pump that would not prime itself".
That is what I would try.
What do the more experienced members think?