70 Mach 1 Turn Signal Indicator light issue,

The 70s have a very odd-ball turn signal circuit. The marker lights are in series with 10 other light bulbs.
I know you changed the cluster lights, and two of those lights are the dash turn indicators. I would replace both of those with the original incandescent lights, and see if that fixes your problem.
The only company I'm aware of that has exterior LEDs that work on the 70 is Vintage LEDs. They have lights designed just for your year.

The dim dash backlighting is the result of the indirect lighting. My 69 shares the same dash, and the brightest LEDs I've found for the 8 backlights is:
Choose a warm white around 2700k and they will look just like brighter versions of the incandescent lights, with no need to even change the blue filters.