The 300E engine is a direct bolt-in & makes 185hp.
The 24V CIS version M104 engine made about 210hp & in the USA was only in the W124 coupe & R129 SL. IT also would be a direct bolt-in but would require some expert wiring & pluming changes.
Any of the later EFI engines would require serious modification to install including all kinds of electronics.

The 260E head WON'T bolt on to the 300E block.

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