It's the 4 digit code on the PCM, something like AOHO or X1A2. It's simply tuned as a supercharged 3.8, he didn't use values from an SC as the base. No need to. Same as putting a supercharger on a mustang. The tune is great and the car runs perfect, it just idles high.
If it is a stock 1990 Automatic SC the strategy is typically GURE. The Catch code is on a label on the processor. C0S is most likely. Most people putting superchargers on a mustang put on vortech or paxton units, not MN12's. The SC intake manifold has different volumes than the stock mustang manifold as well.
For high idle I would start with making sure the idle is setup properly mechanically and that there are no vacuum leaks. There is effectively no difference between how it works on a Mustang vs the Tbird (assuming the mustang is distributorless). The configuration values are likely different since the intake manifold volume, cam, head flows rates, throttle body volumes are different for the SC. (looking at the SC strategy there are 57 Scalars related to idle, 16 graphs, and one table)
A simple google search found this information that may or may not be accurate showing the catch code indicating what Strategy.
GURE = A9U2, C0S(Auto), L0E1(Manual)
GSALC = W1M
GSALI = M2Y, Z1Z2, U2Y, S3S, X1A2
CZAWO = T4J1, W4D2
I don't know if you have noticed or not, but there is no such thing as a simple supercharged 3.8.