You can install the 50 sheet metal on to your 51 as long as you use the entire 50 front end. The mounting points are the same. As the fenders, hood, grill are different shapes so you can’t mix and match. The cab is basically the same except the back window is wider and the layout of the dash is different. The doors will interchange. The rear fenders interchange 48-50 & 51-52 as I think that 51 is slightly different shape than the 50. My truck is a 50 F-1 flat bed. Hope that this will help. Mike