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.