Couple of points:
1) You need the correct cam locking tools to do this job properly.
2) Apart from the belt and tensioner there are two idle bearings. It is ESSENTIAL these are changed.
3) The GTV 3.0 V6 should be fitted with the metal impellar already (this is the waterpump that is used to change the plastic one on the 2.5 V6 fitted to the 156). However, if by remote chance, it is the plastic one, change it for the metal one.
4) If not done recently, or no proof of change, now is the time to change the plugs.
4) Use genuine parts. Don't cut corners.
Finally, and this is important, get someone who has worked on these engines before to do the job. These engines are not straightforward to work on - one small mistake and that's it good bye GTV.