I'm surprised that no one has answered this yet.

You're going to need a driveshaft that was behind a 727 trans (or a 4 spd) and matches the rearend.
I'm guessing that you have an 8-3/4 rear end.

One from a 68 or 69 Charger will be the same, a 68/69 Coronet should be too.

The u-joint yoke that's on the rear end will either be for a 7260 u-joint (2-1/8 inside where the u-joint goes) or a 7290 (2-5/8 inside) which is stronger. But either will work.

I've got a parts book that shows the length but it's in my truck. I'll check it tomorrow and post the length and part number it shows. Odds are you won't find any number on the shaft so you'll have to go by length and u-joint size.