First up, let’s get something clear, while this is technically a “How To” guide - please don’t use this as a step-by-step walkthrough to replacing the cambelt on your own Alfa V6 engine. Not unless you have a high level of mechanical experience and a decent selection of tools (at the very least a ramp!) at your disposal. Changing a Busso’s timing belt is no mean feat, even for trained professionals.
The how-to guide for a home hobbyist wishing to undertake this task could be delivered in just a single word: don’t!