Before putting the top and bottom half's of the fuel head back together I did oil the control plunger barrel and fuel slit o-rings so they would slide and not be damaged while pressing the upper and lower half's together. You don't just press them straight together in a hydraulic press or whatever, you do it by hand and press down on the opposite sides with opposing force so it rocks side to side a little on the control plunger o-rings. They have a little give. It's hard to describe what I'm trying to say in words alone.
This video helped.
It's Mercedes V8 fuel head he's working on so it's different but similar to a 930 fuel head.
https://www.youtube.com/watch?v=HQFhmVHp65o
Around 5:30 he starts pressing the fuel head back together by hand. He's sort of rocking the upper half side to side at first so the o-rings don't get rolled over or torn as the control plunger cylinder gets pushed into place.
Like him I did it by eye with no guide pins and used the bolts to align and pull the upper and lower halfs together once they would start threading in place. If you turn the fuel head sideways before the gap closes up the white plastic metering discs can fall out and you have to take it back apart and start over so I kept it level while pressing them together.
This PDF link for a 928 fuel head is helpful too.
http://www.porsche928forums.com/download/manuals/CISRebuild.pdf