Get another person to help you. Then take the gas line loose at the Carb. Put a piece of rubber hose on the end and place the hose into a clear plastic bottle. Have your helper start the truck and you catch the gas in the bottle, if any is coming out. Bet there isn't. At least not enough to keep it running at higher RPMs. It sounds like a fuel delivery issue to me. JMO

Martin
'62 Chevy C-10 Stepside Shortbed (Restomod in progress)
'47 Chevy 3100 5 Window (long term project)
‘65 Chevy Biscayne (Emily)
‘39 Dodge Business Coupe (Clarence)
“I fought the law and the law won" now I are a retired one!
Support those brave men/women who stand the "Thin Blue Line"! Hug a cop!
USAF 1965-1969 Weather Observation Tech (I got paid to look at the clouds)