Have you discovered the size of the tube yet? I have decided to add a breather tube to mine while the fuel tank is out. I'll hard solder a nipple into the filler pipe. My filler cap has been renovated at some stage with all the internals replaced with bits and pieces that kind of work. Looks ugly and I suspect that it may no longer breath reliably. Early models without the breather pipe must have had a cap that could vent. If my caps breather was sealed, that might explain why the tank developed a leak - numerous cycles of pressure and vacuum inside the tank, flexing the joint. Anyhow, it is very easy to add a fitting to the filler pipe while the tank is out. Can you see the size of the hose from under the car? I figure it must have vented down through the horizontal panel beneath the filler.

Later model fuel caps (on cars with the breather pipe) must have been fully sealing to prevent fuel escaping on the bends.