If you have a 1200 ute & it came from the factory with a two piece driveshaft, then it's a longbed.

If it's a single drivshaft, then it's a shortbed.

Pick up the yellow pages & ring some driveshaft shops. They should be able to tell you if they can convert non serviceable Datsun driveshafts to serviceable types.
I have a later B310 shaft that was converted using Holden uni joints I believe.

This will be a whole lot more cost effective than buying a new shaft.