The 79 SD transmission uses a mechanical speedo cable, the 83 SEL speedo and all W126 speedos use an inductive signal. Swapping an 81-84 300SD transmission and cable might be easier than customizing the SEL cluster to take a W116 or W123 speedo.

W116 and W126 300SDs use a 3.07 rear, a 380SEL uses a 2.47 rear. It'll be slow going off the line and passing but you should get better high speed mpg. Probably intolerable if it's hilly where you are (never been to Jackson Hole). Try the 2.47 then swap in a 3.07 if necessary. 3.07s should be easy to find with the number of W126 300SDs out there.

If the 79 300SD brake booster vacuum hose isn't long enough, obtain a hose from a W126 300SD so you can connect the engine vacuum pump and brake booster. The hose will have a fitting for auxiliary vacuum for the ACC and engine shutoff (W126 central locking uses a dedicated electric pump). There's no way to make a gasser vacuum hose work.

You can use a common switchover valve such as an 80s MB Diesel overboost protection solenoid for engine shutoff. Plumb it so source vacuum goes to the engine shutoff actuator on the IP when there's no power to the solenoid. Wire it so the solenoid is energized when you turn the key to run and start. I've never done this but I read it works.

Sixto
87 300D