Is it over full and where is the vent .

Big block 727 full manual valve body with deep tci pan and 10" converter. Also has external trans cooler. According to TCI trans pan hold 2 more litres or quarts over stock. I have 8.5 litres in it, and i guess i dont know how to check fluid levels? I left it in park, i am hearing i need to check this in neutral. Anyway all is good until i put it in gear, run it through the gears in the garage and the next thing i know i have fluid running out from behind the converter. I was very careful to install the converter, used white grease on the snout and made sure it seated in the pump. I have other electrical issues too so need to sort that out before i can check the trans fluid again.