Ammeter Resistance and Voltage drop

Did you write down the load? The load is everything with a voltage drop.

From '67 FSM
For the tests from Battery to alternator, they are looking for less than .3 Volts when a 10 Amp load is applied.
To have the circuit from the battery to alt output (or reverse) drop under .3 V with 10 amps of current, the voltage drop across the ammeter alone needs to be well under a .1 Volt. The drops occur mostly in the connections as you've noted.
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Side note:
The voltage drop test from Battery Positive to Field side of regulator with ignition ON, engine off, should not exceed .55 Volts. (disconnect leads at ballast for this test).
This is mostly a test for maximum field current through the key switch, regulator and all connections and wires back to the battery with an original alternator, regulator (points), and ignition (points). Alternators that draw more current may exceed .55 volts even though the resistance is within factory spec. Another reason to do test field current draw of any replacement or rebuilt alternator. This FSM does not provide a voltage drop spec to the ignition side of the regulator. It's possible that might be in '70 up FSM or bulletin. I probably looked at one time but don't remember