Plugged up? Possible. Or excessive clearance any where can cause pressure to be inadequate. (including the pump or a stuck relief) Pressure is caused by resistance to flow. Its like washing your car with a big hole in the hose back at the faucet or somewhere else in the hose.
Plugged up is like when you get a kink in the hose, there would also be a lack of delivery.
Hell its been 25 years since I worked on a 383 other than maybe some tune-ups.
The best idea, take it to those “Graveyard Carz” guys.
They only do Mopar.