If the converter isn't locking and a HarryT observed, you are only turning 2350 at 87MPH, you should be realizing about 60 MPG.
If your temperature is really that high, then your engine would likely shut down n short order. Test your leads at the sending unit, again. Determine which one causes the Temperature light to display. Then, with the engine cold, do a continuity test on the terminals of the sending unit. At temperatures below operating temperature, there will be no resistance when probed between the light terminal and the sensor base as that switch is open. The gauge side will show resistance. Be sure your leads are connected to the correct terminals. They can easily be repinned in the harness connector if they have been switched.
If everything is correct, then you have either wiring or gauge problems.