Depending on the extent of the cage, im sure they would have mentioned that they will want all the glass out, ie windscreen, rear qtrs and maybe even rear glass. I had all mine out. 
 
Plus if your going to spray up the cage or whole interior youl want them all out.
 
The more you pull out now, the cheaper it will be if they have to pull it out or slow them down. A full cage will go through the dash, not around it, so you need to either pull the dash out now and then its easy for them, or get them to cut up the dash as required. Id recommend pulling the dash out.
 
Regarding what to put back in, nothing really stock goes back in.
 
Depending what races and classes you want to be in, some need to remain whole interior some require a 'full length dash' so either the top of the factory dash, or I just have a very basic aluminium 'dash'
Obviously if you keeping the factory dash cluster (speedo tacho ect) then the factory dash will be best
 
Use dry ice to remove the sound deadening, i used some old corflute signes ect to hold the dry ice vertically to do the parts on the fuel tunnel ect. 

I'm a bit confused.
 
You say you want to "track it regularly and safely"
 
Why do you seem to believe that you HAVE to have a fully-welded cage installed?
 
There are literally hundreds of people across the states that regularly track cars less, up to and far more powerful than yours.  They use the same principals as anyone else who drives a track regularly - common sense and basic understanding of how to drive safely.  By adding a cage, you are effectively removing the car from any fun you could have with it on the open road (and believe me, I can attest for just how much of a laugh you can have!) and it would just be a crying shame to not be able to just jump in and go for a hoon, whenever the mood/weather suits.
 
You could get a bolt-in half-cage made up (by the guys at AGI, who are doing mine) and pull it out whenever you are not flying around a track.  It's cheaper by a LONG way, doesn't require the dismantling/cutting of the interior and gives you the best of both worlds.
 
I have several friends who participate in Targa events across the country who use this exact setup to ensure they get the maximum fun-factor from the machines and it works.

Don't take this the wrong way - i'm not having a dig, i'm just genuinely interested to see if this is just a knee-jerk reaction to something that you've been told, or if its something that you haven't looked into (or been unable to do so) with a greater understanding of whats going on.

EssDub
 
 
If you are required to run a roll cage, for closed sports and touring cars, half cage wont comply.
 
Rules for cages has changed recently, depending on what your racing in, CAMS / FIA approved roll cage may now have to have alot of bar work, ie windscreen (A piller) support bars, roof diag bars ect, altho some is for national level racing, racing with more than 1 occupant ie rally, targas ect.
 
To have a proper full cage weld in or bolt in (to the same specifications) bolt in would be more expensive and heavier.
 
Also how powerful a car is has NO factor on safety required. Brake failure at 200km/h on the end of a straight is the same in a 100kw or 1,000kw car. 

Interesting points guys, thanks for the comments.
To me personally, I tried tracking a fast road car before, it was very nice to drive but felt like I'm about to lose my licence everytime I drove it on the road.. I only enjoyed unleashing it on the track TBH, but I couldn't push too hard or do too many track orientated mods as it was still a road car.

Lucas
Interesting points guys, thanks for the comments.
To me personally, I tried tracking a fast road car before, it was very nice to drive but felt like I'm about to lose my licence everytime I drove it on the road.. I only enjoyed unleashing it on the track TBH, but I couldn't push too hard or do too many track orientated mods as it was still a road car.

I understand, I lost my licence for six month, I was just testing the boost level, up a hill in a 80 km zone looking at the boost gauge, within 200 - 300mt then slowed down to 80km again I was caught at 35km over, The cop had laser gun right there in that spot. Was not happy, I have not had a speeding fine for over 10yrs.

I think the plan of have a cage fitted then restricting your driving to rally and closed circuit is a good idea. It is too easy to loose your license so it(track only) will work out cheaper in the long run.
To fit the cage, you need to remove everything in the cabin. Have the windscreens removed by a professional. It will cost 150 but worth the insurance.
I would recommend acid dip as it will save a lot of mucking around with welding a chassis covered in seam sealant (I can assure you its a very painful exercise welding around this rubbish) but there is a time and $$ penalty so dropping off a car which is a shell at the workshop then picking it up later is a better option. The lastest CAMS spec tube is a lot lighter than the older products so you will draw back some of the weight gain from a full cage.
Seam welding of the strut towers is a must do.
Doubling up on the spot welds around all the windows is also a nice tweak.
 
Race seat subframe is also a must. Best to have this built so its at floor pan level as its pretty easy to raise the seat but impossible to make it lower than the lowest point. If you are over 5'10", you will need it as low as possible to avoid issues with head clearance.
 
 

Getting there.. slowly..