...on my 84 245Ti

Just returned from a 1000 mile round trip and now ATF drips on the driveway and chassis is wet on the underside. Put the car up on the lift, fully expecting to see a leak from the tailshaft seal/bushing.

Instead, its leaking from the trans cooler pipe fitting where it attaches to the right rear of the transmission housing. It's hard to get to/exhaust pipe in the way. But the ferrule fitting is tight, so it must be the 90 degree pipe fitting that screws into the housing. Whats the best sealant to put on those threads? permatex, teflon tape? Other suggesitons?

Thanks, Robert