Did you replace both cables? The ground side is as important, or maybe more so than the hot cable. A #0 or larger cable to both battery posts is important to have, and the ground cable needs to be long enough to reach one of the bellhousing or starter attaching bolts. The original cable routing of a short braided ground to the frame or firewall and another one from the frame to the engine was simply a cost cutting measure at the factory. It's possibly the worst choice for good cranking, as it adds two more connection points to get loose and/or corroded. Your battery should have enough cranking amperage to maintain a minumum of 4.5 volts at the battery posts after 15 seconds of the engine cranking with the ignition off. Do that crankking test ONCE and let the starter cool down for a minumum of 20 minutes before trying the test again. NEVER- - -EVER- - - -crank an engine long enough for the battery to get low enough that the starter stops turning. The last few seconds of slow cranking can melt the windings from excessive current draw. Starters operate at very near dead short conditions, and the slower they crank, the more damage can be done to the windings. EVERY cable connection including both battery post clamps MUST be clean and tight for a 6V starting system to work properly.
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!