68 dart gas tank leak

You can get this on Amazon by Saturday. I would use my original lock ring for sure with the gasket.
Spectra Fuel Tank Lock Ring LO06
https://www.amazon.com/dp/B001QUTGX0?tag=fabo03-20

Don't toss or replace any parts (sending unit) until your sure they are no good. Mymopar.com will have most of your answers for original equipped vehicles. The search engine here will have hundreds of thousands answers for you. Show us some pictures and give us more info of what what you got. The group will work you through almost anything. Doesn't sound like it's too stock of a vehicle anymore.