Currently we are looking at doing a v8 setup in a Mercedes W115. My one concern is that I want the originals in the cabin to work which includes the Speedometer which is cable driven. No how would you go about ensuring that you can get the same output from the new setup as per the old setup. are there people out there that can make a kit for the actual fit-up and are there engineering houses that may be able to do this??? Please help
If it is just an engine conversion you are doing, then the original speedo should work fine, since the speedo drive/sensor comes off the gearbox or final drive.
If you are changing the gearbox and the speedo drive/sensor comes off the gearbox, then you may need to get a conversion done. There are companies that can do this.
With some Audi conversions, an alternate generation cluster is needed (B6/B7). ECUs need to match the cluster. I'm not familiar with Mercedes in that aspect.
Depending on the swap, if you are changing your ECU to match the engine, you may need to change your cluster. Changing the cluster might make a difference between cable or electronic, or electronic to electronic but with a different signal.
If the engine is the same generation, I expect you won't have any issues. If it's cable driven and you are not changing the transmission, you are good to go.
Edit - I did an Internet search - should have done this first. The W115s were manufactured from 1968 to 1976. As such, you don't have an ECU and associated electronic cluster. Keep the transmission and no problem. Swap the transmission and you may need to swap the speedo gear for correct ratio and perhaps a custom cable (length or connection).