The 318 and the Dart never got along so well. The 318 is reliable enough (outlasts the car), but really needs premium, and doesn’t give the performance you’d expect. Mopars were ALWAYS ball-joint challenged, and the heavy 318 doesn’t help. Also, if you have rust around the torsion bars, you might get a nasty surprise.
The Slant-six 225s varied from year to year, probably depending on the one-barrel Holley feeding it. Let’s not forget the small 8 cylinder 273 that was an option before the 318 was used. The bigger engines (360, 383, 413, 440 etc) save that for the c-bodies and the cudas, Challengers and Chargers (unless you really ARE racing).
Back in 1968, the Iowa Highway Patrol bought a fleet of Chrysler Newport sedans set up for police duty. They were white four door sedans with the 440 motor. I was driving a ‘66 Chevy 396 rat motor and got to know the local patrolman (another story.)
I mentioned to the trooper than the 396 was fortunate to turn in 8 miles per gallon on leaded premium. He said the 440 was lucky to get 10...
As perhaps you are, I am an older model Mopar fan. I replaced a 273 with a 318 in a 64 Dart. However the body was just too far gone to justify any real restoration.
I do have a bare stock 64 Valiant/225 in storage for eventual resto. If you know someone that would like to buy a 70 Swinger/225 stock, no major lower body rust, I have one in a North Ga barn.
But IMHO the reason people love A-bodies (or show fondness) is not the car as much as the engine. The Slant 6 was so reliable you had to really work hard to kill it. If anything was weak it was the carbs but the damn things would still run pretty much no matter what you fed them.
In the eighties and even nineties you really could get a reliable Dart/Valiant for $500 esp if you knew anything about them. However my 64s will be fifty in a few years - thats gettin’ old! Also in Atlanta a car without A/C can only be used about half the year.