Furniture, Wood and Cabinetry Finishing - Built in bookcases reconstructed

Hi. We've moved into a home that we're flipping. There is currently some built in book cases on both sides of the fireplace. On the right at the bottom is a cabinet that is too big for the design so it will be removed. On the left is a bookcase that doesn't go all the way down to the floor (minus about 34 1/2"). Now, the cabinet on the right that will be removed will only leave us about 31 1/2" below the bookcase. What I intend to do is put two wall cabinets under there with a 4" toekick and then paint it all white, wainscotting in the back and then glass shelves with two puck lights at the top of each bookcase. NOW, if you'll notice, there's not equal space for the "cabinet" part on each side. The right side is 3" longer. HERE'S MY QUESTION (FINALLY!): What tool can we use to cut that 3" off without having to pull the whole thing down and then have to reconstruct it? There HAS to be something and I'm surprised to think that my husband doesn't have the correct "toy" to do so. Any suggestions. Hey, he may even score a new toy out of it all!

Thanks,
Flipit