You may not want it to be full. The original tank gas cap is not pressurized.
Gas station tanks may be underground so the gas is at ground temperature.
Here in the South, where temps usually hit 90 in the summer, that cool gas heats and expands a lot.
On original trucks the cab heats up like a toaster oven in the sun and so does the gas in the tank behind the seat.
Gas can force it's way past the gas cap seal and ruin a nice paintjob.
Back in those days you had to remember not to "filler up".

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."