Torynich,
I too am trying to figure out what size wheels to fit on my 240z. I have already installed the 280 YZ and subtle Z fenders.
I am looking at 18x9 wheels with a low offset. Maybe +12 to +20. By my calculations that would require at least a 2" adapter.
What are the other specs on those Cobra wheels? What is the offset or rear backspace? What is the bolt pattern?
Maybe we can put our heads together and get each other's cars looking great!
http://web.mac.com/modernmagic/iWeb/240z