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The rear bumper has nothing to do with it, a Si axle back can clear the non-Si bumper no problem. It's the downpipe. The beginning of the R18 mid-pipe is sloped in order to connect to the downpipe, where as the Si mid-pipe is straight.
the k24's exhaust port is on the rear side of the head while the r18 has its exhaust port on the front. as shown in the diagrams above, the downpipe (or header or whatever you wanna call it) of each motor connects to the midpipe at different points, behind the block on the k24 and in front of the block on the r18. long story short, the two are not interchangeable.