gosh they have done a good job there. it looks great.
can't answer your question about what has had to be changed to turn a impact bumper 911 into a convincing 964.. possably rear wheel arches ??
slady on DDK might be able to help you as he has gone the other way and turned his 964 into a early 911.
don't know if this will be off any interest at all but heres a picture of my 1967 911t and my 964c2 parked side by side.


main diffrence from the front is the length of the bonnet
and from behind


the most obvious diffrence of these two is the bootlids/engine grill/spoiler
BUT obviously yours is a Cab and also started out as an 76 impact bumper so the changes might not have been so great.