Are small and big block A-body driveshafts interchangeable? I am trying to find out if I can use a driveshaft from a 70 340 Dart on a 69 Dart with a big block transplanted into it.

Short answer is yes, IF you are putting in a BB/727 in place of a SB/727.
If it's a 4 speed application, no prob either as long as the same trans and driveshaft will be used in the car again.

The car is currently a slant six automatic. I will be transplanting a big block and 727 using a small block k-frame. I have the 340 driveshaft from a prior project and just wanted to know if I would be able to use it.

Is the 340 driveshaft for a 4 speed or an auto ?

ASSuMEing its for an auto and and 8 3/4 yes ... ASSuMEing it for the same body platform, Dodge and Plymouth have different wheelbase and thus different driveshafts , also the duster/demon dart sport had a specific driveshaft length , not sure what they are the same as.

I don't recall what transmission the guy I bought it from said was in the car it came from. I will take a measurement of it today and post it later.

The driveshaft I have measures 48" center to center on the u-joints.

What car are you putting it in ?