automatic transmission fluid leak(bad)

We bought this car a while ago. The person I bought the car from said he put in another engine, but couldn't get it running correctly. I finally have it running, a new timing belt and tune-up. After about an hour logged driving time, in short intervals, the car developed a really loud rattle and a leak. I have found the rattle, it was the bolts at the front of the torque converter hitting the front of the tranny because they weren't tight enough. The leak starts slow, when warmed up it flows pretty bad. From what I can tell it is coming from the bellhousing. I think it may be the front main seal?????? Input please.

You're gonna have to be more specific on where the leak is. Maybe pics too. It would make sense that the leak progressively gets worse cause the trans fluid heats and losses viscosity.