One of the items that comes to mind is the oil sump, the X5 has the front diff bolted to it and pretty sure it is different from the 335d. Also the EGR is set up differently (but I think you deleted all the emission controls already?). So, yes, sounds like a bit of a wrench fest but not impossible.

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2003 4.4i black ext./black int./black headliner (kid's runabout)
2014 535d X-drive, M package, silver/black interior

Sold but not forgotten:
2009 E70 35d, black ext./black int./black headliner (sold 2021)
2006 4.8iS Le Mans blue/cream int./black headliner, SOLD in 2012 sadly...

Other hardware:
2015 Cayenne S (wife's new DD and definitely wroooommmmm)
2016 Tundra 1794 edition crew cab, luxo hauler
2005 Tundra crew cab, weekend hauler. Sold after 150k very happy miles.