Is the battery up to the task? If it hasn't been tested with the high current draw method, you don't know for sure. Load the battery to 1/2 the cold cranking amp rating for 20 seconds, and check the terminal voltage with the load still applied. A 6V battery should not drop below 4.5 volts to pass this test. Most shops have no clue how to do that test, or what results they're looking for.

For instance- - - -a 600 CCA battery needs to be load tested at 300 amps for 20 seconds. The "battery testers" most parts stores use have a fixed 100 amp load, and are intended to "test" a discharged or poorly-charged battery for the purpose of selling you another battery. The battery needs to have a minimum specific gravity of 1.250 in all cells before it's even charged enough to test. Full-charge specific gravity is 1.265 or so, with a maximum variation of 10-15 points.

Once the battery tests good start getting all the connections clean- - - -no, actually I mean CLEAN! as in surgically clean and brightly-shined with a wire brush. Cables need to be a minimum of #1 and 0 or 00 welding cable is better. Run the ground cable directly from the battery to one of the starter attaching bolts. You can run a #4 or smaller ground strap from the engine to the frame- - - -the heaviest current draw items it's grounding are headlights.

"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!