My fuel tank leaks from the top 1977 chevy pickup

There should be enough fuel in the carb to handle things.
Are you losing fuel in the carb too ? It could have an internal leak if any fuel escapes over a period of a week or two.
The diaphragm in the pump should prevent any fuel from back-flowing to the tank. Same thing with the float in the carb.

Rusted or broke fuel lines.

First of if you have a history of topping off your gas tank STOP doing that .The gas tank is designed to have an air pocket in it.By topping it off you could be sucking raw gas instead of gas vapor into your charcoal canister which will saturate the canister and then you will have to replace it.You may have a leak in the fuel line somewhere Has the fuel filter been changed and maybe left lose or missing the O ring and last possibly a leaking fuel injector

well you leaking gas either from your fuel pump tank float assembly inside the tankand gasket is bad that seals the fuel in or possibly a bad fuel neck aka filler tube also check your fuel lines and make sure all clamps are tight

the pressure in the tanks is not enough to "lift" the fuel up to where the seal is on the top. the tanks usually have the fuel pump and level sensor mounted in thru the top. actually if there was a leak anywhere above the fuel level in the tank it would not leak fuel but leak pressure which you would not see. it most likely is a rust thru spot on the tank, it only needs to be a small pin hole for it to leak and it can be very hard to see.

I'd check for a vacuum leak, the idle mixture. Also the check valve on the brake booster.

Yes, you'll at least need to lower the gas tank to get the line disconnected from the pump fitting on top of the tank.

Removal & Installation To Remove:

1. Properly relieve the fuel system pressure.
2. Drain the fuel tank through the filler pipe.
3. Support the fuel tank.
4. Remove or disconnect the following:
   • Negative battery cable
   • Filler neck
   • Shield and tank straps
   • Fuel lines and vapor hose from pump and tank
   • Electrical connection from fuel pump
   • Fuel tank
   • Fuel pump/sending unit assembly by turning the locking ring (located on top of the fuel tank) counterclockwise using a spanner wrench
   • Fuel pump from the fuel lever sending device

To Install:

1. Install or connect the following:
   • Fuel pump in tank with new seal around opening
   • Tank and connect fuel lines and vapor hose
   • Tank to the frame
     1. Torque to: 33 ft. lbs. (45 Nm)
   • Shield
   • Fuel filler neck and clamp
   • Negative battery cable
2. Refill the tank.
3. Run the engine and check for leaks.

sounds like you are going to have to drop the tank out to inspect and fix. with luck it will be leaking from the fuel pump seal or the return line.
Bad luck would be if the tank is rusted out and needs to be replaced.
Syphon as much gas out of the tank as you can before you drop it , gas is heavy.

You more than probably have a gas leak somewhere in the fuel system. If you can smell gasoline, it is leaking somewhere. The lines run along the frame to the engine. The leak might be hard to see or hidden (on top of the fuel tank).

The leak is robbing fuel pressure from the engine, causing it to idle rough, so is probably on the pressure side of the system.

(Most fuel systems, except for "returnless" systems with Pulse width modulated fuel pumps (not very common), have a pressure line and a return line to the tank.