Help choosing a v8 for my dart

Okay, here's my take:
318:
Comes with the transmission (that's a 3 speed 904 Torqueflite, by the way), torque convertor, and all linkages.
Correct oil pan.
Direct bolt up to 273 engine mounts and brackets.
You will still need a '70 and up radiator, a '66 Dart driveshaft (6 or 8, both used a 904. You need the slip yoke and correct length of the '66 shaft to replace your current ball and trunnion shaft), an early A V8 throttle pedal, V8 centerlink, and 273 (possibly others, many threads about that) exhaust manifolds or headers, and a '66/'67 speedometer cable. Possibly other stuff depending if you have power steering, etc.
5.2:
I love Magnums, but you'll need a lot to make it work.
A transmission, torque convertor, driveshaft, speedo, LA360 passenger car oil pan/pickup/dipstick & tube, intake manifold and carb, all linkages, distributor, appropriate radiator, and all the missing serpentine front drive components and accessories, modified 273 mount brackets to fit the Magnum, and the V8 centerlink.

And both are going to require you to seriously consider brake and suspension upgrades, possibly a rear end upgrade down the road as well.
My vote goes to the 318 at this point.