You can get away without dropping the cage, by the looks of this youve welded all of the front roof bar in. What i did was to weld as much of the main hoop as i could with it in the correct position, then using ratchet straps, pulling the base of the main hoop towards the front of the car bringing it off the little 'ledge' onto the floor giving you enough room above to weld above.
I hope you understand, i havent expalined it to good!
By the way, use a small welding torch, because getting to some of the joints can be hell of a tight squeeze!