You should be able to swap parts between the head and motor, between years, without any problem. I successfully mixed parts from a 70 and a 73.
The 70 is on top, and the 73 is on the bottom. Note the heads are slightly different.
One gotcha I ran into is that the round plate the 70 used is off set slightly. You can see that the space between the gasket mark and the hole itself is not symmetric. If the orientation is off the pump wont work, and will likely burn out the motor because everything gets jammed up inside.
