I've been tying to help you but you keep ignoring my advice on this...

It's a simple if laborious job .

Clean the tank well using the purple industrial de greasers sold every where, I like to toss in a hand full of sharp edged small flat washers, BB's or pea gravel, then fill it about 1/2 full of the de-greaser and water, button it up and shake the bejeebers out of it, remembering to invert it as lots of rust will be in the upper reaches of the tank .

Drain through a large funnel lined with felt and use the strained liquid to re wash it and get more crud out then rinse clean with water and re fill 1/2 way with a dilute mix of Phosphoric or Oxalic acid and water, the dilute mix should be one gallon acid to three gallons water, add the acid to the water, not the other way round for safety reasons .

Use the small sharp edged items with the de rusing agent too, it'll speed up the job and make it better .

Don't use vinegar it will cause flash rusting .

Phosphoric acid leaves a thin coating .

W123 tanks have NO COATING from the factory .

Phosphoric and Oxalic acids are sold cheaply in gallons at big box hardware stores, look in the flooring department .

Sometimes the self service yards don't punch holes in the fuel tanks, this means you get one for about $35 .

I bought the correct 21 gallon tank from a Mercedes Specific junk yard for $100 and was happy with that price, hardly anyone ever buys used fuel tanks .

If you buy a used tank, mind when they remove it as it's very easy to bend the neck when pulling it out .

Only the M-B original fuel tank filler neck grommet will resist Diesel fuel, all the others will dissolve in less than a year .

The in tank strainer is cheap from any junkyard, I'm sure you know there's an LKQ just down the road from you (South) they get lots f W123's in all the time .

The best filler cap gasket is blue, it's available from the M-B Classic Center along with the filler neck grommet and some 7MM fuel hose you'll want to replace at the same time , leading from the tan to the chassis .

Simply peel off the thin sheet metal crimped collar holding the old hose on and it'll pull right off revealing a standard hose barb .

Be sure to use the correct band typ of fuel hose clamp, a # 12 or 13 .

This is all do able easily at home, it sounds like you stumbled onto a jewel in the rough .

Biocide is important, you should use it periodically as a prophylactic measure, not wait for the clear plastic intake screen to clog .

Remember to order in a spare secondary spin on filter too .

In a pinch you can use the in tank screen from a gasser, it has finer mesh though .

I strain re re use the dilute acid mix for years ~ it's handy to de rust parts and hardwre, nothing alloy nor pot metal but all ferrous metals and brass / copper meaning it's handy for vintage carby jets and contacts from unobtanium switches....

Good luck with this, please keep us informed .

Craigslist usually had kids running impromptu junk yards out of their back yards, maybe look there to find the cheapest tank, it should come complete with sender, yours is going to need disassembly and cleaning anyway .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better