That is why.
There are also some short-ish rubber hoses that connect the steel fuel lines, between the frame and the engine compartment; these are well-known to get get brittle and leak after 40 years.
Year of your 40?
Stock F-series engine?
Stock carb and fuel pump?
Charcoal cannister intact?
De-smogged or emissions equipment intact?
Last time the rubber portions of the fuel lines were replaced?