Did you loosen the filler neck bolts? you'll have to wiggle that out of the way to drop the tank, then unplug the harness that runs to the pump, then you can drop the tank.
doin the pump swap is very easy, but when i put the new 155 lph pump in i had to use a hack saw to cut like 3/4in of the metal pipe off and put in a new rubber hose, cuz the pump was longer then the stock. after the wires are unplugged and the filler neck loosed and the tanks on the ground just get like a flat head and a hammer to loosen the screw like ring and the sending unit and pump will have to be twisted and turned in a few directions to get it completaly out. very easy.