If I was gonna use a jack, I would place a timber of some sort under each side and a jack at each corner. Last cab I changed, I got a couple of buddies and we just lifted it on, not to bad with four guys. I've also slid one from one frame to another on a couple of pieces of pipe with carpet on them. Did that one completely by myself, one man can horse a cab around if you have no help.
I'm assuming a 2wd...if not, you're jack has the most lift I've ever heard of...and I need to get one. Now to your problem...are you just looking to lift the body just enough to get the body mount in, or are you looking to lift the entire cab at once? If it's the first, then I wouldn't have any issues putting it right on the rocker. I dunno how the 70's fenders are bolted on, but in the 73-79's the fenders might have some clearance issues and warpage issues due to fender/cab clearance and fender mounting bolts. So just keep that in mind. But I think you'll be just fine lifting at the rocker...i mean I've lifted entire rigs off the ground by lifting at the rocker (in a pinch) with no issues...a couple hundred pound cab won't be a problem.
These are the mounts that are attached directly to the cab, so I'm hoping they will be fairly easy to remove. Luckily the extensions on the frame that the cab mount to appear to be in great shape, minus the surface rust.