Mine's original and it's 5/16.
I changed mine to 1/2" by using a bulkhead fitting and AN fittings on my race car.
You can insert a fitting into you stock tank (any size you want) and use the current sending unit as a retutn line port.
Bad idea. Your return line should always be the bigger of the two lines. That side isn't under pressure like the supply side and needs the help to evacuate.
huh? i've never seen a return line that was larger than the supply line. maybe the same size but never a larger one. the idea is that fuel will be used at a greater rate than it is sent back to the tank, generally.
factory v8 pickups in 69 and 70 for sure were already 3/8". unless you're planning on well over 450hp that's all you'll need.