DiabloSport computer tuner is a good investment:

DiabloSport InTune
The Predator model is being discontinued, and the InTune model is easier to use with a laptop (USB drag and drop files, etc). Data log, view, and tune!

My understanding:
Auto trans got the 3.06 rear gear
Manual trans got 3.73 (18”), 3.91 (20”)
There might be an odd year/option, but still uses one of these ratios.

Remember the transmission gear ratios in the Auto vs Manual are completely different. Look at both 1st gear, and the overdrive gear:
Auto trans ratio : 1st 3.59, 2nd 2.19, 3rd 1.41, 4th 1.00, 5th 0.83, Rev 3.16
Manual trans : 1st 2.97, 2nd 2.10, 3rd x.xx, 4th x.xx, 5th 0.74, 6th 0.50

So an auto in 1st gear might not like a 3.91 rear gear...think tire spin.
For comparison, our old 727's have a 2.45 1st gear.