I thought of something else. I assume you have confirmed that the inline 6 engine is going to fit in an engine bay designed for an inline 5 (or 4)? Will the engine mounts line up to the existing points on the 123 subframe? And will doing so allow enough room for the radiator up front and allow the transmission to fit properly in the tunnel?

If I am wrong and it's been done before let me know, just didn't want you to get into this and discover it won't fit.

I've heard of many 606 to 603 swaps from 210s into 124s but can't recall one with a 603 from 124 into 123.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech