I installed a rebuilt 350 in my 67 and my uncle was helping me get it going. I primed the oil pump, did all the pre-fire stuff.... and the battery I had was crappy. Trying to get the post on better my uncle tapped the positive post with a pair of pliers, than we hooked the battery up to a charger that had a "start" mode. As soon as I hit the ignition (the hood was off the truck) I saw a bright blue spark arc across the post and heard a very loud BOOM. The battery exploded and it blew two of the sides out of it. Thank GOD! my uncle backed away before I turned the key so no one was hurt. The explosion blew the tops off the battery; one landed about 10 feet from the truck but the other went over the garage and was found in the backyard the next day, it blew the sides out and acid was all over my newly painted engine and inner fenders. I quickly cleaned the acid off, we got a new battery, and she fired right up
DON'T EVER MESS WITH BATTERIES!!!