Dump it, wash it out, pressure test it to see where the weak spot is.

Re - weld the bad spot, add a bung for vent.

Use Eastwood's three-stage sealer kit.

Your vent should have something engineered into it to prevent fuel from pouring out in case the vehicle should ever flip over. It can be simple, like looping the vent hose below the bottom of the tank and back up, or complicated like a flow valve.

The ol' charcoal canister plumbed into the manifold vacuum is one of the better emissions changes the EPA forced on us, but unfortunately that charcoal canister takes up a lot of room...