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the 3rd gen tank has no direct vent line, it has a vacuum break line that goes to the back of the rear seat area.

pulling the tank is not too bad, just need a second set of hands and be DAMN SURE you get the car up as high as possible and the axle as low as possible. then there will be no issue with the tank clearing anything or the fuel neck getting caught up. doing this on a lift would be even better.
With no return line, i had vapor lock issues in my car. Usually only when trying to start the car hot, after sitting for a little while. If the lines are insulated where they enter the engine bay, this will probably be a non issue.

I still say remove the electric pump, splice the lines (temporary, should be replaced) and see how the car runs with just the mechanical pump thats already there. Could be a simple fix.

If you run lines, 6an is fine. Factory hard line might be better.