Options for installing AMD Quarter on a '70 Dart

Yes you are correct. The outer wheelhouse tucks inside at the sail panel support. You can either splice it with a lap joint or butt it. The rocker can be spliced with a lap joint at the inner quarter or just peel up the bottom of the inner quarter after you use you spot weld cutter. Either way is relatively easy if you take your time.
I have always replaced mine as the factory intended. If sectioning a rocker I would suggest more than a lap weld. It should have a full backer with plug welds and a solid weld. You can access them for corrosion protection with a wand from the front or rear. I personally wouldnt section a rocker on a classic car... although we do it daily in the collision world it has to be in certain locations specified my the Oem.