Coming on quickly mate!
That looks like a good excuse for a composite roof 

I lost my temper with the sunroof this morning and was quite tempted to chop the roof out!
Where are you going to tag the cage to the shell?
Are you putting in harness bars to the cage?
Are you still planning on MOTing it? New rules appear to state seats have got to be adjustable back and forwards.... might be a bit tricky if you cut the rails out 
Cage is going to bolt in for this year as it'll be coming out again over winter for further works. It'll then be welded in and tied to the A and B posts as well as the roof over next winter.
Harness bar will probably go between the rear turrets but that'll be decided once the seats are in and we can see where the belts will go and the angles.
As for MOT regs it'll be fine as they don't apply to purpose built track vehicles that are road legal.
Nick