if it's the six speed box there are two great gearbox change guides on here, one for the GT and one for the V6 2.5 the procedures are very similar as it's the same box. try search threads for clutch change or similar. the box has two drain plugs one lowers the oil from the shafts but leaves some in the Dif, the other is at the side of the dif and clears the rest out both are normal allen key, huge grub screw type one 8mm and one 10mm. The Six speed boxes take around 2 ltrs I have found mine just slightly over when I put the full two in but there's probably a little residue left that causes that. Filling the oil down the dipstick hole is a pain and takes ages. You can however ( as you'll have the battery box off anyway I should think ) if you buy two of the small 1ltr bottles with the pull out soft extending neck, remove the reverse gear switch, squeeze the neck of the oil bottle or use a big clip like you get on clip boards to stop any coming out then placer the end in the reverse switch hole. In my case it fit so well you could practically screw it into the thread. remove the dipstick to let air out quick and, holding the end hard on the hole and squeezing the oil bottle, shotgun a litre of oil in in around 20 seconds or less, then second bottle, and Bob is indeed your mum's brother. leave about a minute, replace dipstick to check quantity.
Other possible way is to get a length of the thicker clear type washwipe tubing and somehow attach that to bottle and feed down dipstick hole; will take longer though.