Putting a 3.9 Magnum in my 1973 Scamp

There was a build thread a few years back of a member who V6ed his early Dart wagon, can't find it right now; but if it fit into an early A, your Scamp should be no problem.
Decisions and hurdles to overcome:
Oil pan- the V6 oil pans, to my knowledge, were all rear sump. That puts the sump right where the steering linkage is. Some fabrication on the pan and pickup will be necessary.
Engine mounts- more fabrication, nobody makes swap mounts for the V6. Magnum V8s had the LA style mounts in addition to the mid-motor Magnum mounts, the 6s I believe don't.
Exhaust- the manifolds should work, people have been running the V8 Magnum manifolds with a little judicious grinding and engine shimming. The big drawback is that it was manual steering only- won't clear the power box. Now with the V6 being shorter in length, you just might get away with it with power steering. Pure conjecture on that.
Headers are a no-go, nobody makes them for your application, and you can't just mod the V8 headers by lopping off one header tube on each side even though the 6 is basically an 8 with 2 cylinders deleted- they omitted the 2 rear cylinders, (edit- rear on driver's side, front on pass. side) so that would move the entire header rearward by a substantial amount, giving you all kinds of clearance issues. I suppose you could make your own, though...
Electronics- I assume you'll be running the factory EFI (rotsa ruck finding one of the old DC 4bbl manifolds and/or electronic distributors), so start reading up on the computer/wiring requirements for Magnum EFI in the "Magnum Swaps" forum- the wiring and fuel requirements are the same for your 6 as they are for the 8s. Same options for transmissions with the 6s and 8s, too. LOTS of good info there that will apply equally to your V6.
Lots of odds 'n ends I'm not thinking of right now, but you'll have to deal with along the way...
Good Luck!