Very hard to diagnose with too little info. More info needed please
Do you have a breather in the cell? A 1/2" one.
Where is the fuel pickup located? At the back I gather?
Where is the pump mounted? At the back under the car? It should be as close to the fuel pick up as possible.
Do you have a filter between the cell and pump? If so what type? Normal filters aren't designed to work as prefilters. Normal filters are designed to work under pressure. If you have a normal filter? You'll need to buy a good prefilter.
What fuel pressure are you running? 5 3/4psi should be plenty if set at idle.
I had the same issue as you when I used a factory tank with the pickup at the front. Fuel was moving away from the pickup and I was getting a line full of air. Hence me moving to a cell and now I've got a modified stock tank which works very well.