New sending unit leaks at lock ring

You need an OEM lock ring. Even used ones 50 years old are better than any re-pop ones. You could also try this special sealant which actually works and is 100% impervious to gasoline, alcohol, and almost anything else : http://www.permatex.com/products-2/product-categories/gasketing/gasket-sealants/permatex-permashield-fuel-resistant-gasket-dressing-flange-sealant-detail (Permatex® PermaShield™ Fuel Resistant Gasket Dressing & Flange Sealant)