Longer ratio gearbox for hdi

[Dum-Dum]

So just to confirm it's the entire second shaft and the diff that'll need swapping?

Yes, you need to swap both the diff (or just it's crown wheel) and the second shaft in the gearbox. The HDi Eco crown wheel will not mesh with a standard 306HDi box. I know this, I've tried it.

Swap the bits operating the clutch is the easiest option.