Not a Mopar But Ford Electrical Question

Greetings,

I know this is an A Body forum but I really don't feel like Joining a Ford Board for one question.

I have a 1970's era Versatile Swather that has the Ford 200 6 Cylinder in it and went out to try and get it started today and had a good 12.45 Volt Battery ready to go, Got it hooked up positive to postive, negative to negative ground, Once hooked up, the voltmeter reads 0 Volts and the darn thing won't do anything. Once I disconnect the cables, Slowly the voltage goes back up to 12 volts, until I hook it up again. then back down to 0 Volts and nothing. Damn thing worked just fine when I parked it last year and now it's doing this. Cables are all clean, has new clamps on the battery terminals and still nothing.

HELPPPPPPPPPPPPPPPPPPPPPPPPPP