if these old cars have sat for some number of years, first thing to do is PULL the tank, clean it, replace or clean and check the fuel pickup/sender, and check tank for any pinholes while out. yes pinholes! clean/replace the ground strap at the pickup, if an original 71 tank, it would have a vent nipple at inside top side of tank and rubber hose to the vent tube located in trunk. 3 other rubber hoses there at that tube. dirt daubbers?????/
and you cold have a dead Iwegian corn mole dead in the filler hole!