The exact same thing happend to me when I converted to a C4. I took the car to United Speedometer in Riverside, CA (I lived there at the time). They made a gear box that mounted to the back of the speedometer head to get the proper (well almost) reading. It actually screws to the back of the speedometer head using a fitting like that on the end of the speedo cable and the speedo cable plugs into the other end. As I recall it was a bit noisy. I was looking for something the other day and happend across it. Interestingly, when I switched the car to a 5.0L with AOD the speedometer read pretty acurately with out the gear box. Give me a call at 402 461 6080 and we can visit about giving this a try.
thanks to Virgil & sue, The speedometer problem will be addressed. Now, I am having trouble overheating. After getting the car on the road, it would overheat at freeway speeds; but not driving around side streets. I checked the timing advance. I found it to advance to about 32 degrees BTDC at high RPM. I put a stronger spring in the distributor and that fixed the normal driving at highway speeds. Today, I took it out to where there is a good size grade and it overheated going up the grade. I had the radiator cleaned out when I first got the car. My question now is does anyone know what the advance curve should be on the 53 Mercury with the original 8ba engine?