Rudy,
Are you sure you can change the crown wheel from a 4,08 to 3,25 or even 3,46 and still fit it in the 4,08 pumpkin; there's quite a difference in size between the 4,08 and others? I would not have thought that possible. Who has advised you that this is doable?
A complete 3,46 axle from a donor sedan would set you back around $3-400 I would have thought. You'll just need to do some digging around. There were plenty of the things built. I went from a 4,08 to a 3,46, but was unaware that it was possible to simply swap the crown wheel, etc, hence my initial question.
I wouldn't worry too much about the datacard numbers too much. The key ones are the ones stamped on the soft top cover, bonnet and transmission mount in my book. The original engine would be great, but I can't see anyone getting too excited about a different rear axle, unless your car is concours, in which case I can imagine that there would be an impact on the price, although I know nothing about cars with this level of originality. I have all matching numbers, but will keep the old rear diff (probably until I get bored of lugging it around every time I move...).