Cleaning stock Mikuni carb

[FMS88]

Hmm, mine looks similar to that... But also looks slightly different. I am curious if maybe they tried to modify it or something. This is what mine looks like underneath where that ball and cylinder is.

Yes. It's definitely been "modified." The small valve is missing and I bet the ball was jammed in the valve's guide to prevent a vacuum leak. This isn't good. If someone tried to disable the MCV, what other modifications might they have tried that could compromise the carb? If you can't get the Weber soon or you're intent on trying to get the Mikuni to work, plug the rectangular opening in cylindrical part of the MCV valve body with JB Weld (visible in your latest pic) and the other smaller holes with RTV, then reassemble. That should plug the passages from the air horn to below the secondary throttle and any vacuum leaks caused by disabling the MCV.
The MCV has a very limited purpose; it only opens momentarily when the throttle closes quickly to prevent an overly rich mixture. If the rest of the carb is fine, it should start and be drivable despite an inoperable MCV.

I overlooked your post #6. Why was the truck sitting so long?