As you install the new battery, before bolting the cables on solidly, just lightly touch the ground cable to the battery post after installing the insulated cable. If there's a big flash of current with all the circuits turned off, leave it disconnected and start looking for a short somewhere. It could be a cable rubbing on the frame, a shorted starter switch, or some other source of high current draw.
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!