I think Magnum 3.9L is very doable, any issues would be the same as a 5.2 or 5.9 swap, except for the mounts if they are different (are they?). What I want to do is swap a 3.6L Pentastar and 8-speed trans; 292 HP and 30 MPG hwy in a big ol' Chrysler 300, who knows what it could do in a little A-body.
I had 1967 Chrysler 300 body in good shape years ago but i didn't get a motor with it. It would have looked awesome with a little V6 magnum setting under the hood! Probably slow but real cool!