Step 4: Make a Clamp
You'll want a 2"-4" section of 1" pipe to use as the slider on the vertical stand pipe. You can just buy nipples of the right size, but pipe-on-the-hoof is a lot cheaper (assuming you have a chop saw or some easy way to cut it to length). Make sure the ends are deburred, both inside -- so it will fit over the 3/4" pipe -- and outside-- so you don't cut yourself. The before photo below shows two possible deburring tools. You can also use a grinding wheel to smooth the outside and get a headstart on the inside.
Note: This part involves welding. If you do not have access to someone who welds, you could use a 1" tee fitting in place of the 1" pipe as a slider (if you are lucky you should be able to find one that has 1" through and 1/2" on the tee). However you will have to drill the threads out of the straight-through part of the tee in order for it to fit over the 3./4" vertical. This is probably a 1-1/8" drill, or a quick boring job on a lathe. Both of these options require some heavy machinery. If you don't mind a sloppy fit you might use a 1-1/4" tee fitting and a bunch of bushings to adapt to the 1/2" pipe I use below. If you use a tee fitting, then all you need to do is thread a short pipe nipple into the T-leg and skip the next paragraph.
OK, the rest of us are welding then...In the photo I show the union welded directly to the slider, however this requires welding cast material so I'm changing the design a bit. Instead we will weld a short pipe nipple to the slider and thread the union onto the nipple. To do this, start with a double ended pipe nipple and cut it in half so you have two threaded-on-one-end pieces. The length of the pieces is up to you and your creative clamping designs. In any case, grind a notch in the un-threaded end of the nipple so that it fits around the slider pipe more snugly than just the flat end, and then weld it in place to make your own tee fitting.
To attach the clamping bolt I drilled and tapped a hole in the slider opposite the newly attached tee. Then, to strengthen the threads I put a nut onto the clamp bolt, threaded the bolt into the slider, and _lightly_ tightened the nut to the slider surface. Then I tack welded the nut to the slider. Hopefully you will still be able to un-screw the bolt when you are done...You might have to chase-tap the whole ensemble to get it to thread nicely again. To make it easier to tighten the clamp I bent the bolt by wanging on it with a sledge hammer while holding the threaded end in a big vise. Your technique may vary.
Before threading the pipe union onto your new tee, drill a hole in one face of the enclosing nut and weld a 12d nail into the hole to make a convenient tightening handle for the swivel. In accordance with the bad-first-design principle I put the union nut on the outside, but I think it makes a bit more sense to put it on the inside. That way you can easily swap clamp types on the end of the swivel without having a bunch of the nuts clanking around. But either way works...
On the other end of the pipe union we can attach any of the gripping opitons we have dreamed up. Here I would make another modification to the design in the photo and thread the second half-nipple into the union. This way we can weld the clamp to non-cast metal more successfully. See the photo of the vise grip clamp for one example of attaching the clamp to the pipe. In that case I cut a notch in the end of the pipe with my chopsaw, filed it wider so it fit around the clamp, and welded it all together. Again note that many C-clamps are cast steel, so the welding may be tricky. If you are not welding you could notch one side of the pipe and drill through pipe and clamp to bolt them together. Another attchment option would be to use some industrial epoxy, such as Ten-Set or PC-7, but you should make a good mechanical fit first.
And there you have your first third hand.