Welcome on the board -- looks like a terrific "barn find" car.
Any 1968 or later slant-6 engine will work. There is no such mixed "could be either" year as Frank has in mind. 1967 was the last year for the small crank counterbore and torque converter nose. 1968 was the first year for the large crank counterbore and torque converter nose.
If you get an engine from something other than a '68-up A-body (Dart, Valiant, Duster, Demon) or F/M body (Aspen, Volaré, Diplomat), you swap the original oil pan and pickup pipe, and the 1976 engine mounts, for physical compatibility.
Some of the changes over the years were upgrades; others were not. Mid 1976 (not 1975) was when the cast crankshaft came in, and 1981 was the first year for hydraulic lifters; Frank is correct that neither change affects interchangeability.
You can do a couple of searches on
www.car-part.com (searchable used auto parts) to see about a working used engine to run while you're working on the car's original; some searches likely to turn up results include 1970 Dart, 1976 Valiant, 1980 Aspen, and 1985 Dodge D100 truck.
Definitely get the
factory service manual.
_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.
