Replace the tank. Find someone parting out a 300D and buy the tank.
Forget PNP, even though they sell the tanks for (I believe) $39, they list that they "May Have Holes." Nothing may about it, they all do.

they punch big holes in them with a Digging Bar to drain.
Forget the 80 - 83 240D, they have a 17 gallon tank. 79 and earlier and all 300D`s have a 21 gal.
Remove a IP from PNP, they are about $50. Remove the fuel line that comes up from the Feed Pump, and push the Primer Pump, a little fuel will come out, see if it is Diesel or WVO.
Might be a job to do, but remove the Fuel Lines from the same car. Front to rear. Or you can make your own.
Federal Hill Trading Company Easy to form and won`t rust. may be a bit pricy though.
Charlie