Finding source of transmission leak

[bawfuls]

This is the 5-speed on a '77 280z. When I pulled the engine last month there was trans fluid in the bell housing, so I presumed a front seal leak. But when I removed the front cover, this is what things looked like:

 

 

Here's the cover as well:

 

 

I replaced the front seal, but the old one looked fine and the input shaft wasn't even really covered in oil, so I don't think that's where it was leaking. To me this looks like it's the gasket that's failing, but I wanted to check with the more knowledgeable folks here for confirmation. Is there anything else I should inspect or replace here before replacing the gasket and assuming things are fine?

 

I also stupidly drained the fluid before removing the fill plug (which is not moving now), so any suggestions about how to refill the trans without removing it from the car would be appreciated. I think I saw some people have filled it through the shifter hole?

Edited May 14, 2018 by bawfuls