Installation of B&W 683 into a wall cubby

Have set of 683s with Pioneer elite sc07. Moving to new house and building custom entertainment cabinet. Was told I could put a set of 683's into a wall with 1 1/2" space left on both sides and on top, with 6" rear clearance. Looking for feedback on whether or not this will detract from their performance.

Just make sure that they are supported well, that the base isn't flimsy. And being in a "cubby" may make them more boomy, so if you still have the foam inserts that go into the ports, you might want to experiment with those for the best sound (especially if you have a separate subwoofer). Other than that, just make sure that the front of the speaker is as close as possible to the front plane of the cubby.
If you're building the cabinet, or having it custom built, consider allowing a little more room on the sides to slip acoustic foam or panel inside the cubby.
There are some great in-wall speakers out there. Could save you some expensive cabinet work and the worry of how they will sound. Most speakers prefer to breath, they sound best out in the room somewhat. At best they are going to sound different.
The only issue with using the cubby in this manner is that you are limited to using the B&W 683 or speakers of that size until you stop using the cubby.

As for performance, the speakers will sound fine ... you will lose things though like separation. I use Spendor SA1 monitors housed inside an entertainment cabinet. It's not perfect, but it works out well enough.