If I found a 1993 grand Prix, Lumina etc. with a 3.1 and used it's ECM it too would need the prom changed because the 3.1 didn't come with the 4 speed auto trans that my car has.
I've looked at the ECM for sale and the info states that the prom etc. has to be changed/ taken from the original and then installed in to the new one.
I will take a look under the hood and tap on the ECM to see if there is any difference and if not I will then order a new ac Delco ECM.
Thanks for your help