Almost 100% certain its a 4R75W as used in the 2003 Mach-1, Nich. Not that its relevant to the problem! Whether it is a 4R75W or 4R70W, it MUST have Mercon-V, not any Dexron or normal Mercon even, else it WILL fail and very quickly, according to the American mustang "gods" on the forums. Worse still it would appear that all 12l of the proper fluid has been removed by changing the fluid in the wrong way.
David