75 duster slant six weak spark

For the coil test you can indeed do this with the coil + connected and the key in RUN; you will be testing the coil, ballast, and ignition system wiring so that method does not isolate the coil by itself. As for the grounding of the coil -, disconnect the existing wire from the coil -, and then use a jumper (what you call a 'secondary ground wire' ; i.e., another plain piece of wire), connect it to the coil - and ground the other end. Each time you ground and then unground the other end of that jumper, you should see a good spark. As noted, don't be hanging on to the jumper anywhere! (You can connect the end of the jumper to a screwdriver blade and touch the balde to groudn whiel you hold the handle...)

You can also test this while someone holds the key in START to check the coil operation off of IGN2; just disconnect the yellow wire from the start relay and it will not crank when the key is tunred to START.

Yes, you can ohm test the brown wire as you say; make sure any wire you test is completely disconnected at one end. But the latter part where you describe voltage testing along all point in the wire is adequate and tells you more. Put the - lead of the meter on the batttery - and keep it there as you move the + lead of the meter from point to point and write down the voltages. You can do this in START and along the blue wire in RUN.

BTW, do you think this has the original spark module? If so, that is a real 5 wire unit and needs both sides of the ballast resistor in place.

OH, yeah, have you cleaned the ground connection to the spark module reeeealy good and make sure you have perfect metal-to-metal contact between the chassis and the metal case of the spark module? A poor ground there will sure stop it from working; that is a basic 'do this first' type of step with these problems.