It's definitely a 70's Cab at the very least, may not be early 70's, but its 70's for sure. So is the bed and doors.
Starting in 1981, they changed the roof design (square back instead of rounded), and added an additional body line on the fenders, doors and quarters. That's how you can tell all of the sheet metal is 72-80 Early style body parts instead of the later 81-93 style body parts.
Also, the frame/Front Crossmember doesn't have Body Mount Holes in it (there would be 2 holes, side by side in the middle of the crossmember if it was a newer frame), so that tells you its an early 70's frame (pre-75). Early 70's trucks had the Core Support Body Mounts on the Outboard Mounts on the frame. This caused problems with cracking in the core support due to flex, so sometime around 74 they moved the body mounts for the core support inboard to the Front Crossmember. But Dodge never removed those Outer Mount Brackets from the frame (even up to 1993) even though they were never used again beyond 1974-75.