Running on after the ignition is turned off can be for several reasons, usually timing is out, but can also be from build up of carbon, or a weak fuel misture, but I'd look at timing first, the best thing to do these days is set it where it feels best by driving and adjusting, as could be much different from where it's supposed to be.
The leak from the box is from the front seal, this is where the torque convertor goes into the box, a big drum looking affair, easy job to do, but caution as no weight must be suspended on the convertor, apart from that it's a case of take box off, pull out straight the torque convertor, take seal out, can't remember if these have a housing with the seal in, but whichever it is easy to do, just be sure to put convertor back in nice and even, after oiling seal.
Don't know if you are into sporty ?, but you can get a shift kit and adjustable vac can for these boxes, which will improve no end, as will a smaller engined torque convertor as it will have a higher stall speed.
tabetha