David makes several good points but I must question the issue of C.A.R.B.
I do not live in CA but I have read quite a bit about these conversions
prior to making my own decision.
David wrote:

"After the engine is installed in your Vanagon you must take it to a referee
station where you will have your Vanagon reclassified as the donor car and
you will have to pass emission regulations the same way the donor car would
have to from that point on."

I do not believe that this is the case with the Subaru conversions.  A
fellow from the Subaru list wrote this and I have shamelessly cut and
pasted it below:

"You can get your KEP 2.2 subaru conversion smogged at ANY smog check
shop. You don't have to go to a referee even if your local shop tells
you they can't do it.
The shop needs to check through their supplementary list, I'm not sure
what its actual name is. The KEP conversion is listed with substantial
detail. I saw this list at my local smog shop. You may have trouble if
you didn't use KEP cat & muffler."

Since this fellow has been there / done that, I have no reason to doubt the
truth of his statement.  I have also never heard of reclassifying the car
as the donor.  I expect my '86 Syncro to remain a syncro when it is
finished but perhaps that is just another CA oddity.

Mike Pidcoe