can you feel the lifters compress by pushing down on the push rods? Do it somewhere away from the problem lifters first so you know what it feels like.
The factory belt set ups may also be different. Which may mean different crank or fan pulleys. Ask for pics of power steering and manual steering belt set ups for a stock 68 Dart
There are a few rebuild services that will rebuild your box to factory Firm Feel specs (originally for cop cars, emergency vehicles etc) or even tighter for better road feel: Firm Feel Inc. in Vancouver WA, Red Cap in the Seattle area are two I've had good luck with. It really improved the...
Check the interweb. Sometimes this is a well know defect in factory paint for specific colors and the factory will fix it. I had the medium metallic blue on my 1993 Shadow ES peel and the factory paid for the dealership to repaint it.
regarding the vacuum port in the rear runner, that port is also used for vacuum for the heater and AC systems when the controller is a push button vacuum block. Used in many late 60s/early 70s cars with AC and many early 60s cars that have push button heater controls matched with push button...
I believe that you can reach the speed nuts on your back in the trunk, above the wheel well. The vinyl top trim in my 70 Dart is in about the same place and there were even holes in the trunk hinge support that made getting to the most difficult ones easier. it ain't easy but surely better than...