Gas tank leaking on turns but not when parked

[Mike87]

87 turbo, set up 3 years. had tank cleaned out and fit back. all seemed good. drove back from FL to SC last week. On the way back, though, filled up tank for first time. I was expecting the gas pump to just automatically shut off or even come out the top but instead when full i see this gush come from the bottom of the car. It does not leak when parked. It seems to leak when accelerating away or pulling in my driveway, turning etc., which makes me think it is something toward the top of the tank or coming down from filler. Any ideas? Thanks!