The Zender Challenge wheels are not 'non-standard spec' wheels. These were an official upgrade for the 156 and were advertised as such in the Alfa Romeo Accessories brochure for 1999. Alfa Romeo used Zender for most of their upgrade accessory items at this time, especially for wheels and exhausts.
The Alfa Romeo Part Number for the 156 17" version of the Challenge option was: 60877531.
As already suggested, the correct tyre spec is 215/45/17, and this is also stated in the brochure.
Hope that helps.
If you change the alloys from what was original specification, whether they were an option or not, they are non-standard. When I did a similar change on my 146, my excess went up £250, and my premium went up £150 (You can imagine I didn't renew with them!). You have to inform your insurers, insurance is a contract of 'utmost good faith' and if you breach that good faith, your insurers are perfectly at liberty to walk away from the contract - usually just when you want them to co-operate. Even when you buy a brand new car, you have to inform them of all the options fitted to the car - because they affect the insurability of the vehicle.
On the plus side, some insurers are quite ok with this, others less so - and it doesn't depend on how much you pay. You have to
check with your insurer.
Hope you're still insured!!
146 1.6TS, 156 2.0TS, 156 1.8TS, 147 1.6TS, 147 2.0TS, 159 1.9JTS, 147 1.6TS, 159 1.9JTDm, Mito 1.4, GT, Giulietta 2.0JTDm, Giulietta 1.4tb, Giulietta 1.6JTDm........