I think that your best bet is to go down to your local field and find someone who owns one and see if you could take the measurements. This is your best option since you will get the measurements that you want instead of the measurements someone else took. The second option would be to click this link:
http://customer.cessna.com/on the righthand side of the page is the single engine support stuff. Try the service center locator to find a service center near you and if it is close call them up and explain what you are doing and they may tell you exactly what you want to know or maybe even let you stop by and see it for yourself.
Hope this helps in some way,
Kevin
EDIT: Thought I would save you some time and looked up the service center for you. I am almost positive that this is the nearest one for you.
Flightline Aviation Ltd.
Dunedin Airport
Dunedin, New Zealand
Phone: 64-3-486-2754
Fax: 64-3-486-2700
Email: info.dunedin@flightline.co.nz
The only other center in New Zealand is:
Flightline Aviation Ltd.
Ardmore Airport
Cessna Lane
Auckland, New Zealand
Phone: 64-9-299-6710
Fax: 64-9-299-9770
Email: info@flightline.co.nz
Hope that gets you closer to getting your shell built.