I don't know either. You would need someone that had replaced those specific hoses to be tell you what they used. I'd say a lot of guys never even bother with them. From what I remember of them in my 72 Skylark Custom, they are smaller hoses, not 3/8". So if the 1/4" is too small, yes, try 5/16". Even if that is too tight, stretch them, make them fit. Go to a parts store that sells bulk vacuum line by the foot, buy a few sizes, and try them. I'd measure the metal line first to get an idea. When you get it figured out, post the sizes here for the next guy.
It's interesting you bring this up - I have a leak at the farthest passenger side pipe off the tank when my tank is full and I'm parked downhill. I've tried to fix it maybe 4-5 times with various hoses, sealant and clamps with no luck. the hose is seemingly an odd size. So, following.
That's what I got. https://www.amazon.com/EVIL-ENERGY-...8&sprefix=evil+energy+5/16,aps,87&sr=8-3&th=1 10' should last me a lifetime Also got 3/8 and 1/4 to replace everything else.
The tank, RobbMc sender, pump and filter won't either and the stock steel line from pump to carb wont work anymore. Do I have to keep my hood down at shows now?