I'm not sure I understant the following statement..."Also, there could be other hidden problems in the ign circuit, ie, ign sw itself as in burnt contacts between battery and ign, or, a dirty electrical connection at the battery junction block or horn relay buss bar or firewall connector. Current going through a resistive joint heats up, provides more resistance and reduces output voltage through the same interface." Gracias.
Follow the red lead from the clamp, fusible link, the short section from clamp tothe junction block, yes, the little 1"X1" block. Fusible links can corrode inside the insulation, any lumps in the insulation? Broken strands within or at any crimped termination.
The interface of terminals laying onto a mounting surface can be a high resistance joint, wire brush clean and reassemble.
Follow the red lead from the battery junction block across the radiator support to the horn relay buss bar and see there is another orange lead on the same bar. Orange lead is a fusible link. Disassemble and wire brush, check insulation for lumps - corrosion, reassemble. From the hr buss bar, there is another lead giong to alternator and to the firewall connection. Another place(s) for resistance. Once inside the dash, the red lead is crimped and splits, one leg to headlamp switch and fuse panel, BATT, the other to ign switch.
Any one of these connections can be resistive. Generally, a voltage measurement from the battery post to most any circuit in the electrical system, there should be less than 0.5 volts difference. Any measurement more, and a resitive joint is in the path. Same is true for negative side of circuits. Got to have a good return path also. Alt case should be grounded as well, there is alot of paint between the neg post and case.
Tip: connect the ground cable to battery to the block rather than the alt bracket, less resistance. Alum intake manifolds, if equipped, do not conduct current as well as iron.