Does anyone have experience installing an oil pump in a Oliver 550 diesel?  When I took this one out, it had a damaged gasket between the pump and the block’s mount.  I’m not even sure there should be a gasket there.  Also, when I try to install the pump, there is a gap between the pump and the mount.  I know that the top end of the pump is slotted to mate with the keyed receptacle near the cam.  I believe I made that slot, but there still seems to be about a 1/8” gap.
Any words of wisdom would be greatly appreciated.  
 P.S. I attached a picture the does a little better at explaining the problem: