I have a '75 GMC (full sized squarebody), and body mount kits are available from multiple vendors. I would be surprised if Energy Suspension or one of the other vendors does not sell a complete kit for your truck. You could probably also get a complete OEM set from Rock Auto.
Many replacements use polyurethene mounts because they hold up longer, but they can squeak and groan until they settle into place. Lubricating them ahead of time helps with that problem. I was able to replace mine by jacking up the truck body near each mount (2x4 under frame) and replacing them one at a time while parked in the driveway. The most common issue is a rusted bolt that won't come loose, or breaks when you try to remove the nut. On my truck it also took some innovative wrench angles to get to mounts for the radiator support.