Is the actual R8 spindle on a 1J and 2J head the same or are they different? If different then how?

They are the same as far as I can see. My manual shows the difference between the J and 2J heads is from the back gear housing upwards and the parts diagrams for the quill part of the head is shared for both J and 2J.
You can grab the manual HERE (http://mysite.wanadoo-members.co.uk/hoodsmachiningpages/index.html) if you want it. Its zipped but still around the 8MB size.
Hood