HDI gearbox on XUD. Which HDI box?

[Dazecoop]

Looking for a HDI gearbox to fit onto my XUD D turbo for improved fuel economy. Looking on eBay, there seems to be more selection of HDI gearboxes from cars such as 307 and 407? I notice a couple of listings from 407's which are 6 speed HDI gearboxes?

Would any HDI gearbox fit just as easy as a 306 HDI gearbox, or must it be from a 306 HDI?

Would this 407 6 speed be beneficial, or perhaps even better at reducing motorway rpm's?

Thanks!