I've included 2 views of the 75-78 gas tank pick up tube filter.

You're cruising home on I-5 in your restored 78 280. You've got about a 1/4 tank of fuel. For the first time in weeks traffic isn't too bad. You enter the exit ramp, heel and toe from 5th to 4th and then from 4th to 3rd all while turning hard to the right. What does this have to do with the construction of your gas tank?

At the bottom of your gas tank is a large basin and at the center of the bottom of the basin is the pick up tube. The bottom of the basin, in the middle of the radius where the shape changes from horizontal to vertical, are wide slots. With less than a full tank there has to be a way to contain fuel near the pick up tube during hard turns. Without a system like this the fuel would slosh away from the pick up tube and your 78 would stall. 

70-74s don't need and therefore don't have the basin system as the carbs each have their own little gas tank called float bowls. You go into a turn, the fuel sloshes away from the pick up tube but you don't stall because the quantity of fuel in the float bowls is adequate to sustain life.

so that is what it looks like. Thanks for the pic.

You have a partially or mostly completely plugged pick up tube filter in your fuel tank. See my comments about this condition elsewhere on this site.