It won't work. The Cobra tank has a larger opening to fit the dual pumps. The Cobra tank is also baffled differently on the inside. Don't try to force it in because you'll damage a good Cobra fuel tank hat a level. You will have to stick with the GT tank and add a 255 lph pump, or search for an official 2003-2004 Cobra tank.
Goto leathalperformance.com. They have everything you need if you want a dual set up in a gt tank. There is a tab that has to be removed on the GT tank to fit the hat. The cobra tank has a baffle system that the GT tank doesnt have. You will also need a cobra fuel level pick up as well.
Yep, SaleenGT2001 is correct. Dont listen to those other guys except for the lethal performance option.
Since you already have a GT tank and an 03 cobra hat, its simple. Start by removing all gas from the tank. Then pull the tank out and break all the plastic pieces. Once done the hat fits right in perfectly. Some prefer to build there own baffle (if you search the forum you'll find some who have). I did this exact same thing. I saved over $400 buy not buying a Cobra tank. Honestly there is no reason to have one if you can control your foot. I never floor it with less than 1/4 a tank of gas. I do about half throttle at most. Some prefer to not floor it until they have at least 1/2 a tank thats up to you. I've floored it with 1/4 a tank many times and once I did it a little below the 1/4 line. I have 500rwhp and the fuel supply was fine.
Some say that you cant wait to fill up when the low gas light comes on anymore because its missing the baffle. I've actually tested my car and I know for a fact I can. I'd test yours if you want to ever get that low. I only tested it so I know i can go further for cheaper gas station (not arco or BP lol) if needed.
To test just keep a gas can in the car and drive till the light comes one. Then drive another 20 miles or so and fill it up. If you have problems then do drive that low again. If its fine your good.
Just remember use that throttle slowly when below 1/8th a tank though. If you make your car need to much fuel it'll suck that regen of the tank dry and you'll end up having not fuel in your engine.
To be totally safe. Always fill up at 1/4th a tank. I do this all the time. I did the test to let me know how far i could actually go if i had to for some reason. An example would be the closest gas station is 40-50 miles.