I owned a 68 Plymouth Valiant and a 78 Plymouth Volare; both with slant 6's; really nice engines.
The 68 had the smaller 170cu in engine; still lots of power and 27mpg. Keep in mind, that is 40 year old technology, and we can barely get that mpg today out of many engines. No smog controls however.
The 78 Plymouth had the 225 engine; drove thousands of highway miles with it; absolutely bullet-proof engine, really. Of course, really simple too; carburetor and gas pedal. About the only thing electronic that could burn out was the voltage regulator.
These are fun cars that anyone can work on, only problem of course is rust; They used a torsion bar set up; problem is that the back end of the torsion bar was anchored to a sheet metal crossmember of sorts, really part of the body. So, when they rusted out, and they did, the bar would pull loose from the sheet metal. Happened to my 68 but I continued to drive it a few more years, being a poor student and all, I just sat a little crooked. Then the other one went out, continued to drive it, but now it felt like I was in a wheel barrow running downhill all the time. Gave me the impression of more speed, so it was OK; it just didn't handle speed bumps too well and had a clearance between the front end brace and the road of about 5-6 inches as I recall. That became my skid plate when going over railroad tracks.
Last time I saw it the individuals I sold it to were drinking Tequila and shouting something out the window in a lanquage I didn't understand as they drove by my house one evening.