Question about Ford front axles

Why are some Axles 48'' spindle to spindle and some are 50''?

 

When you add different year brake set ups Lets say a 50 inch axle with 48 ford truck brake set up It will rub the outer fenders on a 32 -34 truck .I know this because I just bought a narrower axle .Also different year cars call for difference width axles .

 

Here is a link that I have posted before that shows the measurements for different years.
http://www.ozrodders.com/forum/viewtopic.php?t=13788&sid=0218341c0143e5492733db12dd1912a8
Just like a lot of other cars Fords got wider as years progressed.