That 225 slant 6 is stone ax reliable, for an engine of that era.
We owned a 72 Demon with the 225 slant six. Bought it new & drove it daily for 35 years. Sold it to a collector in 2006 with the original engine intact and not using oil. The tranny did get rebuilt along the way though.
Biggest problem with the Dusters and Demons was leaky bodies. Most of them got rusted out long before the engines gave out. We kept ours garaged and still had to stay on top of it.
One time my wife was about 2 miles from home and the oil pressure sending unit blew out along with most of the engine oil. She drove it home with the oil light on. After I stopped swearing I replaced the unit, filled the car with oil and drove it for another 10 years. I made her swear to pull over at once if the oil light ever came on again.