Engine dies

[67gmcss2]

My truck dies when I unhook the battery. it's 67 gmc if that helps. have had the alternator checked at two places- autozone and advance - checks out fine. truck runs with a new battery (figure it's draining it as we speak). runs fine with jumper cables hooked to another vehicle prior to battery change, but the minute you unhook the battery cable it will die within seconds. any suggestions on where my first place to check is for problem? all lights, electrical, etc. seems to be functioning fine until it dies. thanks in advance for any help.