the first thing to do when looking to build a cage or rollbar is check with the organizer (scca, nhra, nasa, etc etc) of whatever you plan to compete in for their rules as far as roll bars / cages go
some require cages to be bolted in
some require cages to be welded in
some require certain bars at certain angles
so first thing i would recomend is to find out what is required for what you plan to do