tyre size on 17's
 

I am thinking of fitting 205/40/17's to some 17x7.5 cup2's

Dose anyone know how much the tyre sticks out (width) from the wheel rim?

What is the most narrow 17" tyre available?

Just trying to sort out this rear fender clearance issue I have :)

205s or even 215s should be fine. The 205 will stretch a bit more on the rim. If you are concerned about the tire sticking out too much under the fender go with the 205s.
The front you should use 205s for sure since you do not have wider fenders.
Alex

My tyres are Semperit 205-40R17 and fit real nice on the Cup 3's
they actually roll in a bit on the rim and definately the best tyre for clearance. I used to have them on my Corrado and was surprised how quiet they are for a directional tyre