Why 10.7 volt input at ‘71 Duster instrument panel?

Consider this... The starter gets its power through a large gauge wire with very little compenents and/or connections to lower the voltage to the starter. Everything else is supplied by a much smaller wire with a crowd of things in its path to degrade the current path. As soon as ignition switch goes to run position, some things begin drawing that voltage. So start at the beginning. Have a good 12 volts at both sides of bulkhead connector. With that you should have same 12 volts amp meter (across its shunt) and at every branch of black wires leaving a weld splice. One of those branches goes to ignition switch. See full 12 volts going into it the turn it to run to see what voltage loss happens at the blue and other wires coming away from that switch.
One thing to know about voltage paths and draws... There is a blue and a brown wire at ignition switch commonly known as start 1 and start 2. I can never recall which is which. Both connect to the ballast resistor. That resistor doesn't know or care which direction current flows through it. So with ignition switch in start position where on only one of the two should be hot, you will find lowered resistance voltage feeding back to the ignition switch on the other wire. Same in run position where only one should be hot in run, you will find resistance voltage feeding back to ignition switch on the other wire. This does not indicate a bad ignition switch. Many ignition switches have been replaced by owners who considered this back feed across ballast resistor to be a fault in ignition switch.
Factory service manuals with their wiring diagrams don't actually detail how current can travel connections to ballast resistor in both directions.
One more point to make, most instrument panels before 1972 had 2 of hot in run wires at their round harness connector. We know the blue one feeds nothing more than the gauge voltage limiter. The other (black maybe? I forget) feeds nothing more than the brake warning lamp in inst' panel and I forget where this wire originates. You might check voltage on that one also.
For 72 models they determined only 1 was required to power both features of the inst' panel so the blue one was omitted. Same wire that feeds brake warning lamp also feeds inst' voltage limiter via printed circuit board.
In my 67 model, I did add a relay mounted on the firewall to insure everything on the blue wire under the hood would get its full 12 volts without doing any diagnosis to prove the need for it.