OK guys I had my car appraised by an "approved or profesional" automobile appraiser.
He charged me $100 for the report which documents the entire overall condition of the car (for example rust, paint condition, glass condition, door dings, weather stripping etc etc), then he made a note of all the mods done to the car, my car is decently modded (engine, brakes, suspension, interior) so I had him writing for over an hour in my garage.
I then sent a copy of this report to Haggerty, that way they can see that everything is legit by a impartial 3rd party and not embelished by the owner...
Alternatively call Hagerty and they can fax or email (pdf) a work sheet where you fill in the mods you have done and the overall condition and then they needs lots of pics of the car to back it up.
My insurance for the year is cheap $450 and the car HAS to be garaged.
I would recommend them, very inexpensive and the way to go if you value your Z. Pretty much all the classic and sports car guys are going this way.
Hope that helps.
Yasin