Probebly it still had the old style 85-140, thick as tar, if you put the new 85-140, or 85-90, or thinner back in, it will leak faster. But the seals may be orig, hard and maby cracked. If you have the old ones out, napa can get new ones.
Likely you had a leaky transmission before you changed fluid. The level had probably leaked down below the seals long ago. You topped it up and found the leaks again.
If you over filled the transmission it will do exactly as you describe. Did you fill untill the fluid was level with the bottom of the hole, or did you fill untill fluid was running back out of the hole then quickly put the plug back in?
The seal at the back of the transmission isn't too bad to replace. You have to pull the u-joint assembly off the transmission splines, dig the old seal out of the bearing retainer, and very carefully press the new seal in (I usually use a big hammer, a really big washer and a bit of pipe; tap gently). Reverse for reassembly.
Are you sure it is leaking out the transmission input too? If you changed motor oil at the same time you changed transmission oil, you may have found an engine rear main seal leak.
If you have the engine out, you should probably rebuilt the transmission at the same time. It isn't that big of a job. The trick to help reduce the front leak is to replace the front bearing during the rebuild. Get a bearing that is sealed on one side and open on the other. Put the open side toward the gears for lubrication. The sealed side will help keep gear lube from running past the bearing.