Not having them in front of me, and just guessing from the independent fronts I have looked at and remember...
This is what I was thinking:
The front frame rails are really narrow on an AD truck, and so is the track width.
The independent front you have needs to be narrowed enough to make the track width reasonable, is this 6-10" or so?
By narrowing it you may close up the space above the cross member that the engine likes to occupy, then the engine either has to be mounted really high above the front suspension, or it just won't fit?
Again those are just my guesses,
What does your tape measure tell you?
I like the straight axle idea too.
Grigg