mach 460 rear deck question

I have a question that may or may not sound stupid and i need an answer fast. will the mach 460 rear speaker box/sound board....whatever the thing in the back that the mach 460 5x7 speakers mount in from a 99-04 mustang fit in a 98 mustang. i would assume they would but im not sure. if so would it be recomended to buy one of these to put aftermarket speakers in if i get a sub. im thinking that it would be benifical so the sound pressure from the sub doesnt mess with the speakers. I NEED HELP!!