I installed the "K" styled head (back plate). Also, make sure you have a super accurate torque wrench if you have to swap them out. I barely tightened a few of the 5 bolts and pulled the threads clear out. I had to heli-coil all 5 bolts. I am happy I did this, but it was an extra chore and an extra $20 bucks for the gasket kit and heli-coils, and bolts. The "K" head has the #8 and #10 male threaded ports. (I think that is the size). The "standard" head on my 508SD was a no-go for a number of reasons. It might fit other applications, but looking at the "K" head, that is probably the better way to go.