hey i had the same setup you are talking about -
255's to 285's
my truck is a 97 so i hope it applies to you too. heres the quote and link that helped me out --
"Call Dan @ T&C ford tell him you need a 18 tooth speedometer gear that will work with a 4r70w transmission."
http://www.fordpartshq.com/
that website is the same one that i ordered my VSS gear from. the guy there is very nice and helpful
dirtyd0g is the members' name who gave me that information back a while ago... he was a great help. search his name and "VSS gear" if you want the actual forum conversation.