IPD used to sell modified housings with exchange, not sure if they still do. I have seen the rear seal housing sold by several of the Volvo parts vendors with or without the seal installed. You can buy the seals separately and have the housings machined if you know of a competant machinist. The rear housing is fairly easy to machine and can be done on a lathe, the front, I think, would take some carefull fixturing on a milling machine.
The front seal is pretty easy to replace in place, almost not worth the expense of having it modified. When it leaks replace it.