The loop is the issue, right?
Yes, If you fill your tank to the point where fuel comes out the vent indicating the tank is full then the fuel that is in that dip in the hose is going to stay there, effectively blocking off the vent. It's got nowhere to go.
Looks like you have your original tank. No rust issues?
Yes, original 1967 tank. It's got some rust along the lower rear seam and tends to have a slight weep but it's not leaking to the point where fuel is dripping out of it. I think the inside of the tank is pretty clean, I don't think I'm getting any garbage in my filters. Like you mentioned to remove the tank would be a major undertaking, either split the hull or cut the tank up while it's in the bow. Not something I'm looking forward to either.
A couple of things I am looking for.
Front hatch cover. I dont think I can fix this pile.
I have a spare hatch for a '68 V174, same color as yours but you have the redesigned deck (1969-1972) so I don't think it will fit properly on your boat.