What is completely draining my truck's battery?

We have a 1980 Ford F100 Flareside. We just bought a brand new battery and the alternator is fairly new. We have been working on it so we haven't started it in about a week. No lights left on, no anything left on.

Today we hooked up the wires to the spark plugs in the correct firing order for a 351. We then tried to start it up. Husband put the key in and the headlights turned on and immediately dimmed out completely. While trying to crank it, it didn't even click or attempt to turn over.

We tried to jump start it. Nothing! Not even enough to turn the lights on at all. Made one click noise.

What besides an alternator could we check. Ive heard of parasitic drain, of course we cant find our meter in order to do that. But, battery is completely gone. Brand new

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