Umm...IIRC, a 300D engine won't go straight into a 300TD. The 617 used in the 300D's (and 300CD's and SD's) doesn't have a spot in the cylinder head for the self-leveling suspension hydraulic pump.
So, I think you'd need to swap the heads at the very minimum. There may be more, but I honestly don't know. I don't know if the "standard" suspension from the parts car could be swapped into the wagon, although if it were feasible, it might be cheaper than fixing the self-leveling setup.
I would venture to assume that the rear end (I assume you're referring specifically to the differential?) is the same, except that you'll have a different gear ratio in the '79 than you will in the '82 (pre-'85 turbos have a 3.07 gear ratio; nonturbos, I think, have a 3.90 gear ratio), meaning that you'll accelerate somewhat faster, but will have a higher cruising RPM.
Hope that helps some...good luck!