Both dusters n darts came with slant sixes, and 318 V8s. They also came with 340, and 360 V8s depending on the package that was ordered. 340, and 360 cars are fairly rare as they were hi performance models.
The 318 V8 cars are more common, as they were a basic V8 and not a performance engine. However a 318 can be made to perform. It is basically a detuned, and smaller cubic inch version of a 340 and 360. So stuff like cams, headers, intake manifolds, cylinder heads all swap. Im sure more stuff interchanges, but thats a quick list off the top of my head
The slant 6 cars are the most common, as most people bought these car as basic transportation, and in this case for them a six with automatic transmission, and good mileage was what they were after.
Weather its a dodge or a plymouth, they both used the same six and V8s, so its just a matter of finding what body style you like best..
The slant 6 is a great good mileage engine, and it wears like iron. You cant kill one of these things, however performance wise its a total dog. The amount of cash you have to put in one to make it run like a V8, you could put into a V8 and have even more power. Im sure all the 6 popper guys would disagree. I have had slants in the past. Great engine, last forever, i usually pulled em, and did V8 swaps tho.
If you ultimately want the car to have a V8 id recommend you buy one from the get go. Swapping a six out can be a pain.
Good luck with your search.