Classic Army makes a .308 rounder. It apparently still takes a standard V2 mechbox. I believe there are only a few around, so if you want one you better start looking in the ASC classifieds.
However, I think it would go without saying that if you want a gun that is playable for a long period of time you should think twice about upgrading it to just under field limits.
Now, out of this light, if you want to get max fps - get a Systema... or go with a pro-win gearbox; however both are very pricey. A cheaper but yet still a good solution would be to look at getting a complete systema standard V2 mechbox, just as Gryph said. A pre-built of course. With less miss matching parts the better, especially in regards to higher stress caused be high fps.
Just replace the piston and you're set. Take look on ASC and there should be tons of stuff there talking about what to put into guns to get max fps. It just may take a bit of looking around.