A little late to the party but let's see if I can help clear up some confusion.
Pickup tailgates bolt on to K5 by design from GM, not by luck or modifying anything.
Something not many are aware of is soft tops were a factory option in '73 and continued on in '76 with half cab body until the soft top was discontinued in '84.
With soft top, the roll up window tailgate wasn't needed and the soft top option triggered a pickup gate (part# 467209) to be installed on the truck.
No need for window tailgate with a soft top. There was an entire weather stripping package with special brackets that went on the truck at the rear floor vs the weather strip on window tailgate.
Here is a GM diagram of the factory soft top for K5.
And the hard top window tailgates swap right back on. Very simple except they are heavy. Get a friend to help.
Looking at your pictures, the Blazer tailgate strikers are there. The Blazer tailgate hinges bolt on from underneath. You'll need the cables and the tailgate torsion springs as well. The bumper needs to come off to remove or install. That posted diagram shows exactly what you need. Minor differences between power and manual tailgates. Any Blazer/Jimmy gate will work.
And the tailgate is Blazer/Jimmy only. And as someone said, Suburban tailgate is very different.