Options for installing AMD Quarter on a '70 Dart

Just my 2 cents worth- I agree, you did it the right way. Anytime you can avoid excessive welding and subsequent filling/finishing gives you a better final product. As far as where you splice the sail panel, it doesn't really matter IMO. The factory decided to put the joint where it is for 2 reasons- ease of production/assembly and viability of replacement should the need arise. "MoparMike1974" said it, I would never section an outer rocker. They are straight forward, and somewhat structural.
Nice work.