The speedo may not read correct after a swap, but it can be recalibrated. Simple process of marking out 52' 9.5" on the street, remove speedo head, attach hair pin (or other indicating device) to cable end and count the revolutions over the distance marked. It's all outlined at http://www.paspeedo.com.
Found out something had been swapped on my 4.5, causing about 10 MPH difference at highway speeds, as the speedo head was stamped 1.19 for the ratio, but actual reading was 1.225.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.