Hi, when I repainted the Scura rims I sourced 83 degree stems from an Ebay seller, http://stores.ebay.com/kurveygirl, they were the 8.3mm ones because I couldn't get that size here in NZ, hence the Ebay purchase, and had no issues fitting these.
The holes in your rim aren't fouled with paint or burrs of any sort? Seems odd that the 8.3mm won't fit as they are recommended for the Brembo rims.
Hope you get it sorted, I can recommend the seller and the stems are top quality and really do make checking tire pressure so much easier.