Try substituting a 12V battery temporarily- - - -just disconnect the coil from the distributor to prevent burning the points while you do a little cranking speed test. You could also disconnect the heavy wire from the starter to the ammeter if you're worried about damaging the gas gauge, etc. Running just the starter on 12V for a short time won't hurt it.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!