Just a guess, but did you make sure that you screwed it on in opposite order? (far left, close right, left side middle, right side middle) to enusre a tight fit?
There was a post the last few days about using a type of sealant to seal the gasket.

The torque converter on my 1981 300TDT has the grill broken, and marks on the unit inside. I would not worry about this. It appears that someone, while attempting to drain the torque converter, broke them with a screwdriver or bar and used the same instrument to manually pry/spin the t.q. to a place that they could see the drain plug.

Sorry, I cannot help on the leak question, but here is the 'sealant' thread:
http://www.peachparts.com/shopforum/showthread.php?threadid=43064

-Brian

__________________
Current: '91 300TE 4MATIC 317k and climbing...
Former:
'81 300TD Wagon 168K "Tank"
'83 240D 216K 4spd manual "Da Bear" (aka best car ever)

"Never sweat the petty things...
and never pet the sweaty things."