On a stock '70 Malibu with a small block, is the factory fuel line size 5/16" or 3/8" ?

Also, is it worth the co$t to purchase a stainless steel replacement fuel line or should I just stick with the basic steel one?

This is the line that runs from the tank to the mechanical fuel pump. Mine finally rusted thru after 39 years.

3/8th... If you want it too look "PRETTY", OR you plan to drive this car for years on snowy salt covered roads, GO with the SS. Other than that, the OM will be fine...

I thought small block cars got 5/16" lines and big block cars got 3/8"?

Mine was 5/16" from the factory.

I think you may be right Greg. It would still be a good upgrade to go 3/8"

If you have to replace it anyway, 3/8 SS would be a very wise decision and would support future mild performance upgrades.