Get some type of bubble level that can be set at an angle. Put the slip joint in the tranny and measure the angle. Now set the rear at the same angle. The tranny should be pointing slightly down and the rear axle slightly up AT THE SAME ANGLE. http://www.carcraft.com/howto/91758/index.html http://www.ls1.com/forums/showthread.php?t=16319 Here are some nice explainations with sketches.
Pinion angles have been discussed before. I won't play search police but it would help. Remember that pinion angle is the relationship of the axle and driveshaft, not the axle to the ground, a common misconception. I'm pretty sure there's a shoebox example on board. Leaf springs like 5-7deg difference at the pinion. We can safely assume your car's lowered, if your motor's at 4deg and the shaft is straight back out of the trans your axle wants to be 1-3deg down at the pinion. For setting it up like in axle swaps you need wedge plates under the spring pads.
Leafspring? Down a couple of degrees irregardless of tranny. You point that pinion up and the car won't hookup on velcro. I think modern mustangs are flat or pointed up and thats all they can do is spin tires - go to the drags and watch them try to launch. They're spinning so bad you could walk beside them and talk to the driver.
Would you care to try to explain the physics of that claim? Perhaps it has nothing to do with a Mustang's pinion angle but other geometry or characteristics of the Mustang suspension.......... It's very difficult to imagine how the pinion's position would have any bearing on the traction of the tires Ray