auto shut off dumps gas on the side

the reason why the filler tube is bigger in diametre ?

drum roll ...


... because nozzles for leaded gas were larger in diametre . Slightly-smaller than Diesel fuel nozzles , and noticeably-larger than the now-common unleaded gas nozzles .

One way to avert this problem ( I notice you live in Ca . We're stuck with the shittiest "clean air" pumps ; 49 state don't have those irritating an completely ineffectual "boots" on the nozzles !!!! ) is to hold the nozzle upside-down , which keeps the pump from kicking-off and spraying gas everywhere ; and listen for gurgling ; stop pumping when the gurgling starts .

1970 & later "A" bodies have 16 gallon tanks ; '69 & earlier have 18 gallon . I think this was an emissions' concession , as EEC ( et al. ) was introduced for the 1970 model year . I believe that the EEC lines ( Charcoal Canister hard lines ) necessitated the smaller tanks .