From what I've read the standard belt is a 93.5" 6 rib ~ so for the 3.12" pulley you would need a 93" 6 rib belt.
Gates make them from what I've just read on my350z.com.
Ok so doing more research this may be the wrong lengths. Reason being Vortech have changed the belt arrangement (less pulleys or something) so the belts now will be shorter from what I'm lead to believe.
Might just have to wait and see what belt comes with the Vortech and take 0.5" off it when looking for a belt.
The belt part number Tarmac listed converts to a Gates belt number K060795 from what I can ascertain. Which is apparently an 80.070" belt.!? Getting confused now.
Well, yes, either wait and see what you will receive or as you said it's got part number and it says 80.070" - just get 0.5" shorter...but you need to transfer it from inches to mm...
So 80.070" = 2034mm
Then minus 0.5"
So 79.5" = 2020mm
So, you will need 6PK2020 belt
http://www.ebay.co.uk/itm/310826609954
Simples
Belts need to be pretty tight to stop spinning and loosing boost... So you can get one smaller belt(to be on safer side), I think next one down is 6PK2013 (2013mm in length)... You can always just return it and get refund if you don't need it