Had the same problem before concerning my 68 Firebird coupe, finally bought part# FT-34 from Rick´s First Generation (USD 59.95). It´s a 69 (!) 3/8" tube and 1/4" return line repro fuel tank sending unit and at least it´s an exact match to the old one that was installed in my car before. Unfortunately I don´t know if this one was correct or changed by one of the car´s pre-owners.
I posted some pics of the new and old unit at Yahoo!Photos, so you can take a look:
