I did the gearbox on my 307 a few months back. Its not that hard, as long as you have a couple of jacks, and some axle stands.
If its anything like mine, then it will make it easier if you drop the subframe.
It took me the whole weekend, but I was doing it on the drive, which is full of craters and slopes downhill. It was pissing it down with rain the whole time too.