Your first assumption is incorrect. The LS engines are much smaller than the Lexus v8's. Remember that they're pushrod v8's not quad cams. Here's a picture of a Ford Coyote motor and a 302. The LS is dimensionally similar to the 302 and the Coyote is dimensionally similar to the Lexus engine.
What is true, however, and the reason it fits, is this assumption: "If the Porsche Quad-Cam v8 fits, then the Lexus Quad-Cam v8 will fit." Remember, that's a big, wide motor you're replacing in the 928.
