73' Midget 1275 rebuild

I am rebuilding a 1973 MG Midget 1275 engine. Right now I am having a problem installin a new oil pump. I purchased the aftermarket pump and attempted to install. WHen I tightened the bolts it created a binding effect with the cam, and the cam will not spin. Thought I had bad pump, ordered another from another supplier, but with same results. Has anyone experienced this problem?? What can I be missing