how hard is it to swap out the slant 6 with a 360.
EASY!
Are the transmissions the same?
NO, You'll need a small block tranny.
Will it bolt right in?
After the Mods, yep, easy
Or am I in for enough work that I should just find a Dart with a 360/318 already in it?
That would be even easier to do and save $160 in the engineswaps.com swap mounts that I used. No need for tranny mount swap or MOD, it'll fit the S/B tranny no problem.