Once it has a cage you won't be able to street register it. There's some dispensation for drag cars using only a rear half cage, but full cages can only be registered in full blown rally cars and then the rego is limited such that you can only drive it during events, to and from events, to and from workshops for essential maintenance and in CAMS sanctioned parades. In other words it ain't a daily driver anymore.

They're called a "club permit registration" which are the maroon coloured plates in Victoria.
There is ways and means to get around building a cage. I'll PM you to discuss.

On with the cage itself. Buy the latest CAMS manual (from CAMS), refer to the relevant schedule (R ?) and it shows you the basic CAMS spec cage and the approved additions and in the text has the tube material, sizes and specs. I think it covers the maximum diameter reduction for the bends as well.

I'd check with a scruitineer before hand, but I'm pretty sure that with a standard CAMS spec cage if you've followed all the data, you can just get a scruit to come and check once it's built.
There's probably some paperwork involved with CAMS in getting the Roll Cage certificaion number and log book.

If you deviate from the book at all, KA-CHING $$$$, you'll need a full engineering analysis and then submit that to CAMS to get it Homologated. The includes if you want to change to Chromoly, or even TIG weld it I believe.

Also, if your fabricator wants to take to your roof with a holesaw to fully weld the front/rear legs to the main hoop, find a different fabricator. A truly decent fabricator will be able to weld it without cutting holes everywhere, I've seen some cages from the east that have had the roof cut, and it's just silly.

Once it has a cage you won't be able to street register it.

dattos aren't MEANT to be nice - they're meant to hardcore - occasional road use only, semi rally / racing cars with stripped interiors, full cages and a pair of webers.

how do we build a approved cams rollcage ? do I get it built then some one inspects it ?
id like to get it built to suit my sss 180b.
but have no idea how to get it certified.
I have someone in mind to build but yeah just wondering on the tech side of things.

There are ways around it. My mates 1600 has a cage and is street registered. He's been pulled over by the cops and everything is fine with them. Might be on his mod plate?

Green200B has forgotten more about 200Bs than we will ever know.

my cage is blueplated for registration in qld.

thanks what about NSW

In WA, if it's got a rear cage, and the rear seats are no longer functional, it can be got past with a bit of stufing around. If it's got front legs, not a hope.
People have been 'getting around' the rollcage registration thing in the past, but now they've gently caressed it for everyone.
There was a mob (I think in QLD) importing jap cars under the B class rally rego and sending them out on the street. All was fine with the original owners, now the cars have moved on, they've come under police scruitiny and the muck has hit the fan in a big way. Now importers can't just bring in cars with an 'optional B class rego' the car has to be bought in Japan and all the paperwork sorted and approved, in the name of the owner, before it leaves Japan or it's only good for parts once it gets here.
Just because it can be 'gotten around' doesn't necessairly mean it should be.