Join Date: May 2006
Location: Fort Bragg, NC, USA
I think I understand what you are getting at... the previous owner/installer plugged up both the vent and the overflow hoses (which is what the two "holes" are for), causing overpressure in the tank as the fuel warmed up and expanded.
The vent hose should go to a baffle inside the tank, probably on the underside of the inner part of the cap. The overflow hose should connect to the inside of the fuel cap, so when you overfill, it runs out of a hose and into the evap canister or the ground (USA vs. EU). As I understand, you have already removed the charcoal/evap. canister and plugged the hoses that formally connected the canister to the intake manifolds (to burn off fuel vapors).
It sounds like you already made the correct fix. You can connect the vent and overflow (outside of the tank) with a "T" fitting and run a single line to below head-pipe level. Just make sure the lines inside the tank are connected to their respective receptacles and you should be fine.