Check out the attached pic I snapped from my reproduction Factory Assembly Manual (FAM) for the Panel truck model 3105 fuel tank.

Maybe of some help.

According to the pic, it shows the tank filler neck with a flange that bolts to the tank. But without being able to see your tank set-up, can't say yay or nay for sure. Best guess is if you don't see such a mounting flange on your tank, the filler is soldered into place.

Per the shop manual (pic also below), you only have a few steps to remove the tank. The first being "Drain tank.".

Should be no problem following the directions to drop the tank past the frame rail once you have it properly disconnected (not a straight down motion).

Be safe!

Dan
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~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)