I just acquired a low mileage 1991 Cavalier RS sedan. I want to swap the fuel pump, as I expect it may be original. Do I need to drop the tank to get to it, or can I get to it through the back seat?

There is no access panel in the trunk. Don't be Bubba and cut a hole in the trunk pan!

Chock the front wheels.
Jack the back of the car up and put jack stands under it at the pinch welds in front of the rear wheel wells.
Take the filler and vent hoses off from the tank.
Disconnect the wiring connections.
Put a jack under the tank.
Take the exhaust hanger and muffler hanger bolts out so it will lower and rest on the trailing beam.
Take the bolts out of the two straps that secure the tank.
Lower the tank and slide it back some.
If the car is high enough off the ground, you can get the tank low enough to take the pump out without completely removing the tank.
If it isn't high enough, you can just slide the tank completely out.