I'm neck deep in a rebuild/ resto of an 83 D250.
Part of it is figuring out what to do with the rotted off fuel line stubs out of the sending unit.
I don't want a Chinese repop that is all but guaranteed not to be accurate.
I did just buy a 30 gallon tank out of an 85 ( supposed southern truck) with a very nice sending unit.
I've had my hands on a few gas tanks and most are rotted off just like my original.
The actual sending unit top sits below flush with the tank surface.
Once the gravel, mud etc is blown out with an air hose every one has a waxy type build up on the top surface of the sending unit. What is this?

Is it possible it is some sort of assembly lubricant or rust inhibitor the factory used?

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Too thick for an assembly lube. The actual top of the sending unit is brand new clean under this stuff, it's probably 3/8" thick or it was before I dug it out

So it probably is some kind of a protectant then. I don't remember seeing it on mine when I had the bed off and painted it. But then I wasn't doing anything to the tank either. Sorry, I'm no help.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Yeah I think it was supposed to be "some kind" of protectant.
The tubes on my original 30 gallon tank and the 20 gallon tanks sending units on my bench rotted out above that level.

Ok next question
Where to find new lock ring for these tanks? This lock ring is plastic, threads in and has "ratchet pawls," so to speak to keep them from loosening up.
Also where to find the vented check valve that pops on on a high ridge of the tank, and has a 3/16 rubber hose that pops onto it

Is the top of the sender steel? You could carefully drill out the old line and push/press a new section of line into the old sender. I remember this method being a thing years ago when people wanted a 3/8 fuel line for cars that never had a large line sender available for them. If concerned about fuel leaks pretend it is copper and use flux/solder to seal the connection.

Yeah now that I have a known good replacement that's what I plan to try.