Too bad you are so far away. I have 4 or 5 of these carbs sitting in my lower garage I'd give you. I've always replaced these 2bbls with a 4 bbl carb & intake.

From past experience, if the top is (or has been) warped, those carbs never work right. When the top gets distorted, I believe it messes other stuff up. You can press them flat, and file them flat, but most still didn't work well afterwards (I rebuilt carbs for a shop for several years).

Just a few comments about your expectations concerning your 360. I've owned a lot of them, and 15 mpg is probably not going to happen, maybe a rare long trip on flat ground a time or two over a 1000 fill ups. Those motors are a 8-12 mpg motors, regardless of what they were installed in, or regardless of any thing you try to do to improve the mpg. I could get near 12 more often with a 4 barrel carb, but rarely got there with a 2 bbl. You can make a lot of power with a 360, and they can built to be very fast, or they can be built to drag a house off its foundation, just don't expect more the 8-12 mpg at best (many were 8-10 mpg).

The fuel sock in the tank could be a problem, as could any hose in the fuel line, or any steel line that looks a little crusty. Any original looking hose or steel line is game to cause a problem. You only need a restricted fuel flow enough to empty the float bowl for a few seconds to make a motor die.

A plugged vented gas cap is as easy to check, remove the cap, leave it off, and see if the motor runs longer. If it does, replace the cap. I suspect your 79 van does not have a vented cap, it likely has a vent line that goes to a carbon canister, but if that vented line is plugged, or partially plugged, you can still test that by removing the gas cap. If removing the gas cap makes it run longer, you need to check your gas tank vent system. Again, any original looking hose or steel line could be the problem, if a vent problem is found. If removing the gas cap does not solve your problem, anything you do with the vent system won't help and would be a waste of time and money at this point. Gene