Speedo not working

[Fitzy000017]

Hi everyone it's my first post here I just bought a 2000 mustang gt the kid i bought it from told me that he put a auto transmission from a 2003 mustang gt in it. My problem is the speedometer shoots up to 150 when I drive it and it wont downshift. I tried putting a 2000 speed sensor in and then the speedometer just stayed at 0 and wouldn't shift has anyone else had this problem or knows what I should do to try and fix it?