Hey oldchevyguy. Have had a issue the same as yours. Takes a little trouble shooting. But here is a few ideas.

First remove the fuel cap and run it and see if that helps. If not continue on.

1)Remove the fuel line from the carb and the fuel sending unit.
2)Remove the fuel sending unit from the tank.
3)Check the screen on the fuel sending unit (remove and replace if plugged).
4)Blow the lines out backwards and if you need run some mechanics wire threw it.
NOTE: DO NOT BLOW LINES OUT WHILE STILL HOOKED UP TO TANK
If all is clear then you know the lines are clean (mine were packed full of rust and I just cleaned them out this way). Blow the sendin unit out as well (mine needed replaced).
5)Inspect inside the tank for rust. (Clean, Repair or Replace as needed)

6)If there is rust found Clean all well and assemble. Run the truck. If this dose not help then there may be rust sucked in the carb.

7)If no issue is found here then continue to trouble shoot the carb.

Good luck with your quest