Just use this. the voltage is adjustable. Works fine (I've used several), but I recommend a little silicone seal over the adjustment screw as water ingress was a problem in early attempt.
http://www.transpo.de/Catalog/E_Cat.htm
Go to >browse catalog>regulator>bosch>IB372A or IB373A
Direct link will not work on the site, so you have to get there by "hand".
FWIW
The Volvo guys use this or a diode soldered in the regulator, as a resistor, to increase voltage output from the alt.