There's another source for that leak that's hard to find, but fairly easy to fix. The driveshaft slip yoke that plugs into the transmission tail shaft has a cap on the back of the splined area with a tiny pinhole in it to prevent air pressure buildup as the yoke slides in and out- - - -a classic example of engineering overkill. Once the converter drains and the fluid level gets above the centerline of the output shaft, that vent hole becomes a drain hole. It will drip constantly and the fluid looks like it's coming from the speedometer cable housing. Pull the driveshaft, dismantle the U joint and epoxy or braze the hole shut. It doesn't fix the drain back problem (that's a leaky one-way check valve in the valve body) but it does eliminate a known leak point to the outside.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!