Really deep box extensions?

[hbiss]

I'd put rigid and bell boxes surface mount and abandon the old circuits.

Don't forget that you can run conduit around on the original siding. There will be a space behind the new stone.

-Hal

 

[Knuckle Dragger]

I have used these before not from HD though.

I would do everything possible to rewire it and install the flush.
Either way good luck