Ok, then if the batteries had a charge , then the short didn't exist before the starter change , ( a dead short would have drained them in no time ) my starter has a power cable and a ground cable attached , BOTH ARE BLACK, so they would be easy to cross up when changing the starter.
Remove all wiring from the starter, and insulate from ground, then hook up the batteries and see if the problem still exists, if no arcing occurs on connecting, then test the cable ( s ) at the starter for correct positive and negative, with a voltmeter, mark them and then disconnect the batteries and hook the cables back up to the starter, and re-connect the batteries, watching for an arc.
There is the possibility of a bad re-built starter, but they are usually bench tested to prevent a bad one making it's way to the customer.
Hope this cures your problem.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.