The pivot points on the D50 and TIB front ends are not the same. The D50 axle is straight, with the pivots vertically in line with the center line of the axle. The TIBs come off the knuckles right at an angle and then hook upwards at the ends even more. The TIBs are also longer, with quite a bit more overlap.

Both of these trucks are stock:





Note how the D50 axle pivot is under the corner of the cross member and down about 2". Compare that to the TIB that is past the end of the cross member towards the opposite tire AND is also tucked up higher.

This shot gives you an idea how much the beam is out of line with center:



The D50 is "square" to the center line of the tire instead of making a "X" at the middle.