I have decided the height of the frame will be 740.

The maximum vehicle height in NSW is 4.6m.

The smallest tunnel in Sydney is 4.4m

My car is already 1.6m

so 1600mm+10mm standoff +740mm frame height + 10-100mm shell

comes to a maximum height of 2450mm.

Well below the legal limit and tunnel heights.

However I will need to watch my center of mass. I am hoping installing a battery bank, water tank and fuel tank as low as possible where the rear passenger seats were will be enough to stop me tipping. 

I would like to possible sit down comfortably inside possibly on a fold out lounge.

I measured my sitting height on a low lounge to be 1200mm.

If am able to extend it out vertically at least 500 which allows 200mm for the linear bearings, guides or however it going to be guided.

In the end the internal roof can be higher this if need be.

I've also have to take in to account the width of the mattress, unless I have my feet going through the trap door.

II am going to cut a 3m length in to 4x750mm