Driveshaft length. Is there a solid answer?

Measure measure measure.

That said, it should be around 49.25". Should being the key word there. That is the factory spec, and it's actually the length of the driveshaft in my '74 Duster with a 340/833/8.75. But just because that's the factory spec doesn't mean that's what yours will need to be, gotta measure to be sure.