I’m adding outlets to my kitchen and have run into an issue. One wall is furring strips nailed to block. Between the furring strip and the drywall I’ll have 1.5 inches between the block and the front of the drywall. I picked up some 8 cu inches shallow boxes which end up sticking out about a quarter of and inch which will work after the backsplash (will probably need some box extenders). After doing research however, I’ve learned that these boxes may be too small for my needs. I have one 12-2 wire going in and out of each box. Outlets will be gfci. I wanted to see if there was anything else I could do short of chiseling out some of the block to accommodate a larger box. Any input is appreciated, thanks!

Some newer homes, our daughters for one, have the outlets mounted on the underside of the cabinets. No wire in The walls.

That’s a good point, I didn’t even think of that. I was actually able to find some pretty shallow boxes that are wide enough to give me the cubic inches I need, thanks for the input though!

The old way was to just take a hammer and break out the block behind it. If on a web, you can move over a little so it falls within a cell. Better is to take a Die grinder and cut out for the boxes, and the conduit. It will need to be mudded in after all is in place.
Since you are have backsplashes, maybe use a Deeper box, I HATE too shallow of boxes installed by others, leaves little room for wires and devices. Same as for switch boxes, Use larger to have room for Dimmers. The price difference is minimal, the Money is in the Labor.

The boxes I found stick out about 0.75 of an inch past the furring strip, so it'll give me 0.25 after drywall, which I'll probably need with the backsplash. I'll be sure to leave plenty of wire in the boxes, thanks!

Whoever does the Backsplash, especially if it Is Marble or Granite for them to cut up to the sides of the box, and so the 2 screws can be inserted. See some who cut openings screws will not go in or GFI will not fit. Then I have to break out a die grinder, Feel it is tilers job?

Thanks for your response, I'll be doing the backsplash and will be sure to leave room for the screws.

AND to the sides of the box. Good Luck