He installs it with the front lining up as it should, with a 2x4 or whatever behind it to space it out properly. Then the gap gets filled in with a piece of stainless steel. Perfectly normal.

I have the same issue and am planning a different approach than what Fori described because I have concerns about the hood sticking out too far. I plan to build a bulkhead to mount the hood under instead of using the cabinet, and then we can mount the cabinet doors on the front of tje bulkhead with perhaps some filler as needed around the edges where the cabinet face frame would be. The cabinet is really useless anyway with the vent running through it. And I think the difference in cabinet depths will actually look nicer than having everything on one plane. In many designs, the hood cabinet actually projects out father than the cabinets on either side, so this is just the reverse.

We bumped our hood out 3 inches. We have a SS rapped wood piece to fill the gap.

thanks for the comments, everyone - I trust someone I'm paying a fortune to will figure this out (and probably charge me even more!).

You want to be sure your hood covers as much as possible of the range area in order to successfully capture the grease, smoke etc. It should come 27" out from the wall at a minimum over a CC range, 30" is better. Farther than that and you hit your head on it. Where it is relative to the cabinets won't really influence performance.