I would suggest that you take the line off the fuel pump first, and leave it in the threaded hole. You must first clamp off the rubber hose that goes from the metal line along the frame to the inlet of the fuel pump. Now would be a good time to also replace this hose, because it is in all probability old and cracking. Then, holding the large nut, with the proper size wrench, and using a tubing wrench on the fuel line where it enters the large carburetor nut, loosen this next. You will now have both ends of the metal fuel line loose, and will be able to move it to the side, to take the large nut out of the carburetor. Once you have the large nut off, you can remove the fuel filter. Usually, there is a spring located at the back side of the fuel filter, and it is important to make sure that you don't loose this, and put it back in with the new fuel filter. The next step is exceedingly important. Before you start turning the nut into the carburetor, place the nut into the threaded hole, and turn it backwards, until you feel it "jump" over the last thread. Then gently turning it to the right, and continue to screw it into the carburetor by hand. If there is the slightest bit of resistance, stop and turn it clockwise, and give it another try. If you ruin the threads of the carburetor, the carburetor will in essence, become instant junk, and a new carburetor will be in your immediate future. I have seen even experienced and seasoned mechanics ruin Quadrajet carburetors by cross threading this nut.
Also, as Dan has said, there might be a filter in the fuel pump, and now would be the time to change that also. From memory, it is a 1 1/4" wrench to loosen it. The filter is a long paper filter, also with a spring behind it. The reason for the springs, is that should the filter get clogged, it will allow gas to flow into the carburetor, so the car will keep running. Usually, by the time that you have a filter bypassing, you will also need a carburetor rebuild.
Now that you have the large nut reinstalled, and gently have tightened it, it is time to reinstall the fuel line. Start by screwing the fuel line into the large nut, but don't tighten it. Go to the fuel line nut that goes into the fuel pump, and install that finger tight. Then give it just enough tightening to secure it. Go back up to the carburetor nut, and while holding the large nut securely with a proper sized wrench, finish tightening the fuel line. Then go back to the fuel line at the fuel pump, and finish tightening it. Reconnect or unclamp the rubber hose, and you are done.
One of our members called me with a leaking fuel problem at the carburetor, and it was a Quadrajet. The shop that worked on the car last, cross threaded the fuel filter nut, and it cost him hundreds of dollars to source the correct remanufactured carburetor for his car.
If you feel that your carburetor needs to be rebuilt, there are a few recommended rebuilders listed in the "Recommended Vendors" forum, or I can give you my recommendation via private message.
Paul