Well Casey.... if you are into making a nice "Frankestein".... why not drop the 300SD drivetrain into the 450SE?
Because the 450SE is quite nice as it is. If I was seriously going to go down that road which would take a lot of work, I'd want an SEL body. In all seriousness though, I'd almost certainly use an OM617 turbo to pull that.
Because I'd want to be able to tow something as well. Can you imagine an OM616 non-turbo in an SEL body with a trailer in tow??
Going up the rocky mountains in my 240D, I maxed out at about 30-35mph - I was flooring the pedal and semi trucks were passing me. Shortly before that, I'd burned a hole through my resonator as the exhaust had come loose, so I was barreling out black smoke like I was running on coal.
If money were not a limiting factor, I'd make a 420SDL by putting the engine from a euro S420 CDI into a W116 SEL chassis. That would be significantly faster than a 6.9, *and* easier on the fuel.
0-62mph in 6.6 seconds, max speed is 155mph (electronically governed, which could surely be removed, and in the W116 there would be about 500 pounds less dry weight).