If it is for flash, read Wilt's post above.
Low ambient condition flash work, with regard to exposure, relegates shutter speed as largely irrelevantly (nitpickers please note that I indicated "largely irrelevantly" not "totally irrelevantly").
If it is for continuous light, you cannot do exactly what you want with the 308 (at lease with my older version of the 308 ), but you can arrive at the same place without a lot of fuss.
You can only "directly" set the shutter speed on the 308, and it will give you the appropriate aperture for a correct exposure. But, after this basic reading has been taken you can push the up and down buttons and the meter will give you many other shutter speed / aperture combination that will give you the same exposure. So, you take a basic exposure reading and push the up and down buttons until you see the aperture you want.
This sounds a lot more difficult and time consuming than it is. After a couple of practice tries, you should be able to get the aperture you want, and the corresponding shutter speed within 5 seconds (or so).
In other words, it is easy. Try it.
Enjoy! Lon