Overall width is the same, but with the side bolsters you lose a little space. This is the 39" bench which is what PRP says fits. They say the 40" doesn't, but there are a few people that have done the 40. I barely had room to move my fingers to fasten the seat in, so if it was any wider I'm not sure I could have. It may be a little different in an LJ and I know at least one with a 40" is in an LJ.
I just sat in the back next to my son's harnessed booster and it's snug unless I put my arm on the back around the seat. You can see that it's tight up at the shoulders. If he was just in a regular booster it wouldn't be bad. But it's only ever him in the back anyway.
I have tumble on both sides and mainly use the driver's. Mine is an early TJ, which should normally be 1" higher than yours I believe. I don't have measurements with the factory seats but I just went out and took these. From the floor hump to the highest point of the seat is about 11.5" and from the low point in the floor to the lowest point of the seat is also about 11.5" in case that helps. These are EndureTrek's with PRP's bracket to mount to the slider. You would still need their brackets or to make your own since that's how you mount them to the sliders.
Gate Keeper make some nice products, but sometimes their marketing isn't exactly accurate. And you don't want to spend the $250 if you don't have to.