Can;t get gas up to carb

I am looking for help. I have a new gas tank and a new sending unit and a new 3/8 fuel line. The only thing that is not new is the return line. The car ran when I had it hooked to a 2 gallon gas tank. Then I tryed to connect to the factory tank and nothing. I shot compressed air into tank and then had some gas come back and I put on the line and it brings a small amount to the fuel filter that I can see, but not enough to keep it running. I poured gas in the carb 8-10 times it would fire and then run out of gas????? This car has sat a long time, the return line the problem? or is it the Vans 3/8 sending unit I put in? Anybpody have experience with them?
any info would be helpful.
T.J.