p71 trans wap

I was thinking about swapping the transmission, driveshaft and rear diff from a 99 interceptor to an 01 Grand Marquis and was wondering what all was involved. Would I need the computer reflashed or could I just swap out the GM computer for the CVPI's? Also does anyone know if the grills will interchange? And is axle code X5 3.27's or 3.55's? I heard 3.55's come with posi and 3.27's don't anyone heard this?