At the yard I frequent, there is a w123 that was manufactured 10/84 So that would make it the 85 model. If I were to stick that rear in my 83, what kind of top end speed would I be seeing @ 3200rpm. Is the switch worth it?

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81 Mercedes 300SD 289k.......SOLD
82 Mercedes 300CD 252k......slow ride

82 mercedes 300 SD...mi Unknown
83 Mercedes 300D ????ksniff..gone too
84 Mercedes 300D 148k........SOLD
85 Mercedes 300TD 386k and holding some one elses project