I did the same with my stock tank. The result was flooding problems caused by high fuel pressure. The small tube/nipple couldn't flow enough and acted like a regulator. I installed a T at the tank and used both connections on the tank. No problems since. In hind sight I probably should have made the connection at the filler vent hose. It's much larger (about 5/8") and can handle the flow.
I just returned from Colorado. While there, every time I took the gas cap off to fill the tank, I got a whoosh of air rushing in. I plan to move the return to the filler vent and plumb a charcoal canister to the small tubes on the tank.
Thanks for all the replies
What size fitting did you try first? Being a plastic tank my vent line is pretty small, ID wise.
I wasn't crazy about the idea of adding a T to fill line.
I ended up tapping the extra boss for a fitting with 3/8 barb. I drilled out inside of barb as far as I could.
There is another boss, tapped already, that I hope to use as a transfer point from side tank (eventually).
Maybe I'l hook up pump, lines, and throttle body to the tank, before
I put the tank back, to see if the pressure spikes.