How did you remove the pump in the first place? Ideally you loosen the backing nut on the top FSS stop bolt, and adjust the bolt a little closer to the radiator (so the FSS can articulate more closed). With that done, the notches in the fuel rack armature line up better with the opening in the engine block that the rack pin is supposed to line up with. Its done this way so that they cant unseat themselves under normal use.
With the pump out of the machine, you should be able to manually move the FSS armature 'open', and with a finger into the opening for the metering pump on the block, feel where the pin notch is in relation to the FSS movement. So when you determine where the rack should be, you take the metering pump and put it back into position... that rotating pin should drop right into the notch if you have it lined up properly. It should sink right in like magic. Then make sure the barb on the pump is pushed 'back' towards the push rod tube as much as it will allow (this will allow for maximum fuel delivery, but make sure both pumps are clocked to a similar position) and then put the cast iron keep back on the pump and tighten it down. Readjust the FSS stop and see if she runs.