Why is this happening?!!Please help.

I run a hose from the fuel pump to a gas can and the slant 6 runs great. I hook up the fuel line to my tank and the dart starves when i drive down the street and up a small hill. It runs out and dies like im out of gas. Ive put in like 4 or 5 gallons, and the new sending unit i put in reads just under a quarter tank. I dont think my tank is pressurized properly. Fuel Sloshes around so bad just moving the car. Are there no baffles in these 63 tanks. Why is she starving for fuel. What can I do besides fill the tank up all the way. I should be able to dive on hills with a quarter tank of gas.:banghead: