Got around to checking what dif i really have. It is in fact 3.9. My brother and I checked by putting the car on jack-stands, marking the drive shaft, turning the wheels together 10 rotations and counting how many times the drive shat rotated.
3.9=39 rotations.
4.1=41 rotations.
4.3=43 rotations.
We know Dave's car has a 4.3 so we used his car to confirm this would work.
We marked the tape with a Sharpie and on my iphone it was very clear, the youtube video is harder to see but if you place your mouse at the notch in the tape it is easy to count the rotations.
My 3.9.
Dave's 4.3.
We took the cars out to play just a little and my car is indeed quicker, but he has a full interior so it isn't exactly scientific. And before it gets brought up, we were not out street racing.