Recheck the filler neck gasket and neck itself for cracks, tears, or just not being installed right causing a vacuum leak.
There can only be three other problems:
You may have damaged or knocked loose the fuel line(s).
You may have damaged or knocked loose the vacuum line(s).
You may have damaged or pulled loose the fuel pump wiring.
When you dropped the tank, did you unhook the fuel lines, vacuum lines, and electrical harness to tank? If not, something may have torn or come loose.