Usually with most cages/ Roll bars you would either cut the area where the bar lands on the floor so you can drop the cage through the floor to weld the cage all the way around or try and tilt the cage and weld it.
It has to all be tack welded before doing any of the above and before the floor plates are welded in. Make sense?
See link below. Hope this helps.
http://www.mig-welding.co.uk/forum/t...install.39556/