I just bought a sifton oil pump for my 56 pan. when i tighten last bolt shaft locks up. I wonder if anyone has any ideas.

 

If the flange won't match up completely face to face when it is tightened it will distort the bore or body jamming the shaft. Check the metal to metal contact with bluing if you have it. Could be your case, or the pump that is not flat. I've seen this once on a lifter body, but same chit with the pump. The tighter the bore is the more it will show up.
Ron

 

Something is forcing your pump shaft out of alignment good chances that this would occur even if it were an S&S pump. My recommendations are to pull the cam cover ie "nose cone" and check gear alignment