You didn't mention if you had excessive tire spin or not. I also don't know how acurate those G-tech units are but I would go to the drag strip and compare times.
Running 13's is definately possible with 5.0 and 4.6 mustangs. Anyone on this board will tell you that.
I bet with GT-40 heads and intake and a 65mm T/B you will be well into the 13's
------------------
'82 GT 351W,BBK shorties,
Comp Cam,edelbrock intake
625 cfm carter carb. c-4 tranny w/ pro stick shifter. 4 core radiator
Mallory distributor. 3:27
.........................
|