I've always gone with the front hubs,, but of course the wheels need to be straight , so i guess that leaves room for error.
I suppose you already pulled the axle, and axle mounts, so you cant get a "before" measurement???
Unfortunately there isn't a difinitive answer to that one. I have measured from anything and everything and have still had to compromise on the measurements. Just start measureing from stuff and see what works.
i have measured from the lower ball joint castle nut.. the center.. usually wont effect the measurement too much if the wheel isnt completely straight.. if at all.
It's not good practice to use a moving suspension part to take measurements from. Sometimes, both the upper and lower arms are adjustable and can give bad measurements. Try to stick with factory stamped holes in the frame or something along that line.
Also, knowing the wheel base doesn't help much either. A lot of trucks don't have their wheels centered in the wheel wells, so blindly trusting stock wheel base measurements is not a good idea.