Vehicle Speed Sensor effect on tuning

Speed density has nothing to do with the vehicle speed. Speed density is calculating fuel requirements based on cylinder displacement and engine speed. The program is set to deliver a given basic amount of fuel at a given engine rpm, and is then adjusted further based on engine load from the MAP sensor, O2 sensor, etc. This a very basic description, and hopefully I did not butcher it.
For Magnum trucks, vehicle speed is used for speedometer, trans controls, ABS, etc. There will be no limp mode on a truck the does not have an electronically controlled trans. Further, the PCM will run the engine just fine without VSS input, as others have mentioned, regardless if it's OBD1, OBD2, electronic trans, or non-electronic trans. Think of it as revving the engine in park or neutral. Same idea. That's why Magnum swaps are easy.