You absolutely want to avoid over filling. If you can't get the fill plug out, want to be safe and don't mind the extra work this is a good solution: The fill plug is 1/2" to 1" below the lowest edge of the output shaft's oil seal. You can disconnect the drive shaft from the transmission, disconnect the transmission mounts and, using a jack, let the back of the transmission tip down until it's level with the fill plug's height.... Now just let is pour out of the extension housing for a little bit.
Not the easiest way to get the job done, but it works.
__________________
If something breaks or you need to contact a member of the administration please post HERE. Unless it's a private or administration matter please post it on the forum. It benefits no one else if car related questions aren't posted for future users and takes away from the time I'm able to spend helping on the rest of the forum.
If you're so inclined I'm always more than happy to accept tips via PayPal. 
Tip Jar ---> 
|