Technically you do not need the GM IAT sensor to run speed density as with speed density the default settings will take readings from the fuel temp sensor. The only time you will run into issues taking that route is during extreme temperature changes such as summer to winter. This is when the GM IAT sensor will become very useful and allow the ECU to adjust accordingly with much more precise temperature readings.

I was able to successfully run my speed density setup off of the fuel temp sensor for 6months before there were notable changes in ambient temperature, at which point I added a GM IAT sensor.