switched 12 volts

Find the ignition coil ballast resistor. One side of it would go to the ign switch, the other connected to starter solenoid/relay and coil.

Well?

I guess one MIGHT, in a WILD stretch of the Imagination, call that a BALLAST resistor, huh?

Sheesh! It's NOT what it LOOKS like, it's what it DOES!

Anyway, good on ya for solving it.

fuse block works for me...

Did you try each end of each fuse? I ran into a similar situation with my 65. It actually made a difference (though I have no idea why). And I was able to track down what I needed for both constant and switched.

Physics. The internal resistance of the battery coupled with the high amperage required to turn the engine over will cause a voltage drop. ...or are you talking about just turning the switch to the 'run' position and you get the voltage drop?

If you're turning the engine over, there's just no way to avoid a voltage drop and the voltage you read could be as low as 6 volts. Of course, the wire in the wiring harness has a certain ohms/ft resistance that could also affect your reading, but it wouldn't be significant unless you were reading the voltage right off the back of the starter itself.