99-04 GT's had 3.27 rear gears in 05 they went to 3.31 with an option for 3.55 I believe. I have no idea on V6's~ but I didnt think they had 3.55's. The 03/04 cobra had 3.55's though stock to help with the gearing on the T-56 6 speed in them.
You need to change your rear end fluid either at 50k or 100k lol not very often~ its not super easy to do but not hard either. You can either suck the fluid out and reload it (through the fill hole) or you can pop the rear cover off and rain it and reseal it and then put fluid in it. The issue here is the reseal HAS TO BE GOOD or it will leak. Its hard to get a seal back on the rear differential cover since oil is hard to get off and get a clean surface to bond with for the RTV so many dont mess with it till around 100k~ or if they are doing a gear change. You can buy gaskets for it instead of RTV silicon.
for your window sticker you should still be able to access it here
services.forddirect.fordvehicles.com/inventory/WindowSticker.pdf?vin=[your VIN here]
just replace the brackets with your vin number (do not include the brackets in the link right after the equal sign put your vin number)
I got lucky and tracked down the guy at ford over inventory and had them make me and mail me a new window sticker
the one above is only electronic but its printable etc