Where can I buy a reliable remanufactured 318 for 1971 Duster?

The mid late 1970s to the early 80s was the Malaise era. No/low effort on the manufacturers part resulting in boring undesirable American cars. Looks like current customer service and final product quality has made current times a type of Malaise era. Buying an engine assembly thru a local Autozone/Napa/OReillys I would imagine gives you a good convenient guarantee if your engine is faulty. Another option that costs more and adds inconvenience would be to crate up your 318 and send it via truck to the closest machine shop you trust. Have them completely rebuild your engine and then freight it back to you.