It's been a long time since I had a 365, which is what I expect you have, but I think you can get the end cover off without pulling the tranny out of the car. I think after you unbolt the rear mounting plate from the frame, you can take the mounting plate off the end cover and by raising or lowering the rear of the tranny I think you'll be able to the end cover off.
Be careful to keep the detent ball and spring for the 5th/R detent that lives under the little plastic cover. I don't know if you can get the spacer plate off.
It's not necessary to use a gasket. You can use a good RTV (room temperature vulcanizing silicone) sealant. Permatex makes a specific RTV for synthetic oils and one for gear oil. When I rebuilt my NG3 tranny, I asked Permatex which one I should use. They recommended the Permatex Gear Oil RTV which I used and it has worked great.
