re:  The regular ones can only be installed when installing
the head, but you can get (more $$) telescoping tubes with
an internal spring that can be installed with the head in
place.

This is NOT true as regards air cooled Vanagons, as tubes
can be removed and reinstalled without removing heads. I've
replaced seals on mine several times now.  The restriction
is true as regards earlier than "type 4" air cooled VWs,
such as old Beetle engines.  I'm not certain, but I believe
statement may also be not true as regards waterboxers.