I have to replace the rubber hoses from the fuel lines to the fuel pump. My fuel tank is about 1/3 full. When I disconnect the rubber hose from the fuel supply line to the pump, will the fuel run out of the tank? Or will I just get a small amount of gas that's currently in the fuel line?

IF you can jack the front of the car up, you'll not have as much fuel come out. You will get some, either way. You'll also get whatever is in the pump, and the line from the pump to the carb. Just get a large drain pan, or some other like container to catch as much as you can, while you swap the hoses.

A trick I used along with Joel's advice was to leave the old hoses attached to the steel fuel lines and use vice grips to pinch the hoses closed. Then remove the hoses from the fuel pump and install the new ones. Then one at a time quickly remove the hose from the steel tube and stick the new one on. You don't end up with as much fuel running out.

I you have mild compressed air, take the cap off of the tank, disconnect the fuel line, and give a gentle shove of air back through the line. 

when i did the 75 i found that taking the new hose and sticking the end of a drill bit to plug it and doing the line coming from the tank first and then connecting the fuel pump, i got less gas all over. when i first got the car had to cut the line a little on fuel pump side (before i got the new ones) and was a mess with all the gas. did hook up the vice grips to the old hose though and that helped to as was said.

How about if I just disconnect the fuel line at the tank. It looks like it is accessible through the fuel filler opening. Won't that solve the problem?

You do need to be gentle when pinching the lines with vice grips.  It's easy to damage the lines and create future leaks.     Done with care, there is no problem.