The the extra length of the 58-59 frame rails does not affect the wheel base or the instructions usually. The instructions are usually uniformed to the 55-59 trucks and they don't use the front of the frame horn for that reason. They usually use the front spring hole which would be the same for 55-59 to make the instruction work for all task force trucks. Like this.
Even if they did use the front frame horns you should never use the instructions like they are set in stone and start welding, do the mock up and put that crossmember where you like it, if you have to move it up or back a little so be it. I only use the instructions as a starting point. I even used a metal tubing as a mock up once when some one rebuilt the front end of the frame and lost of the reference points. I hung the fenders and use a long piece of tubing until it was centered in the wheels marked that as the center line and proceeded.
Here that installed
All I'm saying is do the mock up and make sure its how you like it before you weld it in. Even that 114 wheel base is not set in stone (the rearend could be a little off who knows?), hang those fenders and do the mock up. Mock up is everything. Mock up for looks and measure to make sure its square and you are good to go.