If it's leaking out of the bell housing it's the mechanics fault. It is recomend to replace the the torque convert seal and converter bushing If the engine is out. It's VERY easy to replace these. Pull the converter off. pry the old seal off. fish the bushing out. install the new bushing. install the new seal. slide converter back on and make sure it locks into the dog ears. It is a 5 minute job ....really it takes no more then5 minutes.
The reason you replace that seal is it often fails when you unbolt the converter from the flex plate and push it back towards the transmission. There is usualy dirt or rust the it get run over. If the seal is hard it fails. It's just something a good mechanic always replaces because this is a common issue. We are talking $7 in parts here.
1959 22' Caravanner
1988 R20 454 Suburban.