Guages are extremely sensitive to ground.
Since they work fine, and the engine cranks strong, ground wouldn't be my first thought. That said, extra grounds never hurt.

It looks as if the ignition is supposed to draw power through the ballast coil, and only gets full voltage at crank, that's interesting.

Anyway, chase the voltage.

Do you have full 12V to the ballast resistor?
Do you get full 12 V to coil during crank?

Check the voltage to ballast resistor with ground test lead on engine first, then with ground test lead on the negative terminal of battery. If voltage reads same, then grounds are good.

Test voltage at coil while cranking motor. Test as you did before, with ground lead first on block then at battery.

Another check you can try, is place negative lead on battery negative terminal, and positive lead on block, while cranking engine. If you see voltage then you have poor ground.