50 hp Yamaha big foot lower unit

This is a good lower unit off a two stroke (?) Yamaha 50 hp motor. It is a big foot unit so it takes the same size prop as a 70 hp. the shaft is 21.5 long from the face. The waterpump is included. It has 14 splines and they are 0.620 in diameter. It will not fit my 50 hp 4 stroke. Call Jon