I think it would just be easier to get a 400e. If you have ever been underneath a w124 with a v8 you would see that even the factory had a hard time getting a v8 to fit. Be prepared to beat metal into submission with a hammer.

couldn't you just swap speedos? The instrument cluster pretty much comes all apart.

goodluck

__________________
08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D