65 Barracuda Filler Neck Solution?

The problem the OP is having has NOTHING to do with the tube baffle, or the vent. The tube baffle is to prevent surge / spill when filling

To prevent damage when driving THE CAP MUST SEAL. These caps ARE NOT TO BE vented. They must be SEALED. Also check that the vent tube is clear, which comes out of the filler tube and runs down through the trunk floor. That is what vents the tank during driving