1966 A100 with a Slant gas gussler. HELP!

Before you go tearing the van apart to swap transmissions, fix whatever is causing your rotten fuel economy. Tune-up parts and technique suggestions in this thread -- pay attention to your valve adjustment. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download. I note you have the Holley 1920 carburetor. I've posted before about a Holley 1920 I once had on my '65 that exhibited strange behavior: The engine ran very well and had fine power, but was suspiciously easy to start and drive from cold (zero choke required, just hop in, turn the key and drive off without any stalling or gasping...even with the choke disconnected!) and the car was getting under 10 mpg. Turned out the brass seat ring that retains the power valve check ball, had fallen out of the metering block. Result was that the power valve was always WIDE open. I put the metering valve back together and restaked the seat ring, and things went back to normal (20mpg, choke and careful cold driving required).

Beyond that, check for stuff like exhaust restriction.

Once you find and squash the bug(s) spoiling your fuel economy, some upgrades might be in order. Start with the HEI upgrade .