This video is about what you need to modify on your 03-04 tank to make the tank swap work in 99-01 Cobra or GT.
Glenn's Performance Cobra Tank: www.glennsperformance.com/product_info.php?products_id=218
CJ's Pony Parts Gas Tank Seals: www.cjponyparts.com/catalogsearch/result/?q=+99+cobra+gas+tank+seals