So what did you end up deciding for your cage?
The cage which zzzz posted a link (
http://jeepshots.com/shots/projects/...s/P1000584.htm) to has me a bit underwhelmed. It has nill effective triangulation. The diagonals across the roof won't effectively triangulate that rectangle as it tries to deform, instead they'll generate a big bending force on the main hoop.
Now with the seat essentially sitting 'inside' the main hoop obviously that rules out the traditional diagonal there, but at least he could have moved the cage rearwards enough to add diagonals between the rear legs.
Bogged, if you don't add diagonals then in a rollover the strength of the cage is limited by the bending strength of your steel, by adding the diagonals you convert that bending load into compression and tension along the diagonals for the small increase in weight you get a massive increase in strength and rigidity. The problem usually ends up being space and practicality constraints. Spreader bars are good for preventing intrusion but aren't particularly efficient in strength:weight terms in building a good cage.
Depending on how lenient the engineer you talk to is, you can get away with differeing side intrusion bars. I come from a circuit racing background and as rigidity is all-important, everybody *wants* a double cross type side intrusion setup as this is the most rigid. Some engineers will allow a low double cross (about the height of your thighs while seated, again I'm talking modern sedans not 4x4s) others will only allow one diagonal bar running from the main hoop to down quite low on the front hoop.
Maddog, have you got some photos looking downwards into your jeep?