Slant 6 oil pressure woes…

This is just food for thought. Also I need you to clarify you have replaced the fuel pump and also the oil pump that now leaks. ? I have owned many Slants over the years and have experienced your same problems.
Not knowing the condition of your engine I have to assume it has never been rebuilt.
Lets start with the oil and dipstick. Sounds like your oil pump pickup is on it's way to clogging with sludge.
I have seen some Slants so thick with sludge that the oil light would flicker or come on while driving/idling.
Maybe your traces of black is a tale tale sign.Also rubber valve seals that crumble with age can clog the pickup causing low oil pressure.I have had some that are like a baby's rattle after pulling the pan and pickup and finding debris.
Rusty has said about the smell of fuel in the oil as a well known fuel pump leaking internal into the engine and filling the oil with gas. Be careful, pull the stick out away from the car and light the tip of it with a lighter. If it flame up its gas in the oil. Quality of parts now a days suck so even new,it may be bad.
I think you are destined to pull the pan and see what is inside.
Had a 67 Dart given to me as a kid and went thru low oil pressure. I too started with the oil pump,nope same thing, then pulled the pan to find a clogged pickup. Just my thoughts.