For a '75, you probably won't find any "no drill" solutions from happijack or torklift. Back then they used "belly bars" which are bars that went under the belly of the truck, fastened to the frame, and extended out on each side to catch the turnbuckles. You'd need two. One for the front, and one for the back.
As for the tailgate, leave it on the truck until you go to get the camper. It helps support the rear corners of the box and prevents them from getting fatigued and floppy. You also get better fuel economy with it on, believe it or not.
Oh yes, and one note about the plug... Not all campers use a standard 7-pin wiring scheme even though they have a standard plug. At least in 2000, Palomino, for example, had a proprietary wiring scheme on that 7-pin plug. I had to make a special umbilical cable for mine that connected to the standard trailer plug.