Text below is from the Service Manual (not SRT6) - SRT manual has the same text and pictures of the 160mph dial - page 8J-59! Dash is a complex bit of kit - for instance the instrument cluster includes the FSS which calculates the service interval with information supplied over the CAN data bus from the CAB - Controller Antilock Brakes and PCM - Power Control Module!
CAN data required for the FSS indicator:
4 WSS signals for calculating vehicle speed and distance from the CAB
Coolant temperature from the PCM
Engine speed from the PCM
Load torque from the PCM
Speedo is actually driven over CAN data bus by the CAB. The speed displayed might be from the front or the rear, if the car is cornering it will use the front (an average I suppose?) if car is not turning but there is a rear wheel spinning that will be used as the highest wheel speed is then displayed!
Doesn't help determine if the SRT dash will work, I would guess it might - be ready to connect a lot of plugs though!
I am amazed if a Crossfire dash worked in an SLK - if it does it's a testiment to Chrysler / MB that they didn't need to change any devices so much that they wouln't comunicate! If it did work with the SLK's different wheel sizes I suppose it suggests the speedo change would work.
The instrument cluster includes four analog gauges.
1.
Speedometer The speedometer has a 0–160 Miles Per
Hour (MPH) primary scale with Kilometers Per Hour (km/h)
on the secondary scale. The speedometer receives wheel
speed data from the CAB via the CAN data bus and can be
verified by comparing the speed signal viewed by the
DRBIIIT from the wheel speed sensors (WSS) and the output
seen on the IC. The Electronic Stability Program (ESP)
indicator lamp located between the left and right turn signals
in the speedometer face illuminates when it is activated
by the Controller Antilock Brake (CAB), indicating the
need to adjust driving behavior to the prevailing road conditions.
Also located in the speedometers face are the trip
odometer and the Flexible Service System (FSS) indicator/
odometer (1). To the right and left of the speedometer face,
mounted in the IC, are the rotary dials (2-3) for the FSS
and the digital clock.