someone was saying that the full cages require the removal of the back seat,,,Not true if the cage is designed well,,,the Dept of Tpt approval officers that i use are quite happy to "Blue Plate" our full cages with rear seat still installed. Many times over the years, we`ve had to re-design the cages to suit the changes to both CAMS & Dpt of Tpt rules, but they have been pretty well stabilised for some time now, so we can point you in the right direction.
I suggest both welded & bolted to the floor as both CAMS & Dpt of Tpt want one or the other so doing both supports both Authorities
If going full on CAMS racing then they will require the fitment of a rear half "diagonal" which can be a "Bolt/clamp" in affair so you can remove it for road use after the event - easy-peasy as with the intrusion bars. std issue these days & easy enough to apply.
Norm Singleton is able to make up the bars to order & fit them if need be, no point buying from overseas,,,plenty of local people able to do a better job at lest cost than overseas ordered.
we don`t do the cages anymore,,,too time consuming & too expensive for us to do it, when others, more set up for the job, can do them just as good but at a cheaper price.
There is lots involved with cages for both road & track & homework is really needed to make sure you won`t have to rip it all out & do it again
best to do it from a bare shell to start with
anymore info just give me a call or drop in
easy-peasy
but if it happens and my mini dies, i dont wanna go down with her 
, but unless you are serious about participating in motorsport at something higher than the club level, I'd be putting the money into something else.
made of FIA approved stuff, 
bolt in but could be welded if wanted i guess.