Slant Six Oil light coming on — need a new pump?

More than likely the sludge in the pan is clogging the pickup.
Never did I see that object in a oil filter before and it looks like a spring is in there too.
Somebody must have bought a JC Whitney anti drain back fix all at one point.Take it out.
Mopar Slants had a stand pipe in the pump but in the later years did away with them.
Now it sounds like you do not want to pull the engine or at least the oil pan.
Doubt there is anything wrong with the oil pump other than gunk and sludge.
You can TRY a oil change of 4 quarts and one Quart if Dextron transmission fluid.
Run it for awhile Maybe a day or two.Check the dip stick for a color change (dirty black).
If so the Dextron is working. Change it again doing the same and look for the results.
Slant sixes are cold blooded and many mom and pop drivers never went far enough to get the oil hot to burn off
moisture causing buildup over the years. I'm sure the valves were never adjusted either.Pull the valve cover and you will most likely find loots of sludge.. I like Rusty have had my share of Slants over the years and more than once have done this procedure. It just takes some time.
Slants are tough and will last forever with a little maintenance.