To be honest I can't remember ever doing work on the 355s speedometer sender. I'm trying to think where it would be on the engine, at work right now we have three 355 engines out that I was just looking at on Saturday when I stopped by to check out the silver F50. From what comes to mind, it has a system like the 308 that comes off the passenger side of the trans. The 355 has a transverse transmission (like the 308, looks diff though because engine is now longitudinal making a "t" out of the two). It's the cover that is on the pass. side of the trans. There should be a wire harnesses coming into it to pick up the signal. I would start by checking to see that this harness is still attached, then follow the wires and check them. I don't know the exact routing of the wires, but they obviously have to come under the car and into the cockpit to get to the dashboard. Checking the sender, etc is a good place to start. If all that seems well it would depend on how much of your car you want to pull apart I guess.
btw, maybe you should keep the odo disconnected...low mile 355 10 years from now