Love that Chevelle. I think you're on the right course with much of what you have listed. I like the sump idea over that of a fuel cell myself (fuel cell was more pro street). Maybe hang a large fuel filter back there too. Ladder bars rock on those cars and that's spot on there. I think you're OK with an older MSD. Perhaps a dual point and good wires to finish things off in the engine bay? Tach...definitely Autometer and you gotta have a shift light too.
You can find a good set of 692 Turbo Mufflers or a set of Corvair Turbo's on eBay from time to time. They were the rage for good flowing and excellent sounding mufflers back in the day. Keep us posted on your project as you move along. Nothing wrong with Day III either.