I just got mine running, and I initially over filled it and I blew fluid out the vent tube quite a bit on moderate or hard acceleration. It dripped and hit the exhaust exactly where you describe... I talked to a couple friends of mine who are big Chevy guys and they both said they keep their trans filled only to "barely on the stick" or the "cold" mark when hot to prevent blowing fluid out the vent when hot. I dropped mine to that level and have had no leaking from the vent at all... (I still have a front pump leak though, so it will hafta come out again soon to fix that)
So maybe try pulling some fluid off if you're at the full mark, I did it at the cooler line, cause I can't get a pan under my car at ride height... It also works really nice if you don't have a drain plug...
Joe