I have a full rack on my '72. I originally made it for an '84 and it will fit a 60-66 longbed, too. My first racks were round pipe because I felt the round worked best on a 67-72...and round is equally strong in all directions. BUT... those dang fish mouths are a pain. So I started doing square tube and it looks fine on old trucks. They all have some squareness to them, too.
Here is the full rack. That's angle front and sides sitting on bed rails
I like to use my truck for work some, but I didn't buy it to be a work truck. I had no choice but to put the racks on when my '92 croaked. What I want to make, hopefully this year, is something like the OP wanted. I want a headache rack pretty much as on my full rack. Legs tapered in with top bar just high enough to clear the body and little nubs extending off the legs above the top a couple inches as side stops. I want the back bow to only be mounted when I need it, but want it strong. I'm thinking to do the same base with angle three sides and make a way to linchpin the rear bow to mount. I think I'll incorporate a gusset at the bottom of the bow legs to provide a second mounting point for each side to stabilize it better. Either that, or make two pieces to mount between the front and rear with some sort of quick mount.