It's not a Grand Marquis. The Grand Marshal takes styling cues from several full-size sedans from both entry-level and luxury markets, but isn't meant to replicate any of them (except for the name). The 5 Series and 100 were both full-size/executive cars, which means technically the I-Series (which is a midsize car) are more closely related to the BMW 3 series and Audi 80. [This is incorrect as Youngtimer pointed out, the 5 Series and 100 are midsize cars] And again, the I-Series is not just a mashup of the two. But let's not hijack this thread and turn it into yet another "BeamNG cars in real life" thread.
I stand corrected. Converting between American and European vehicle class sizes is confusing. Repost of the Bluebuck body I put on a Grand Marshal frame because I'm running out of images to add to the bottom of my offtopic posts so they don't get deleted: