Were the line tie downs at the valve covers on? Did you put on a new triangle shaped gasket between the pump and the block? Did you put anything on the gasket to make it stick better or be slippery?
Turn your old pump by hand. It is pretty easy to turn. Now, put a little sideways pressure on it and it gets a lot harder. Add in a possibly rough gasket surface, a drooping IP if the nuts are loose, lines maybe tied down and you with basically no leverage to move the pump. It can be hard to turn. Snap-On and Kent Moore both sold tools to help turn IP's way back when. Some web sites have suggested vasoline on the gasket to help do timing adjustments. Once you get it set, how much more do you plan to move it?