What exactly are you boxing? The 98+ Ranger frames are already boxed at the front half.
As for your plan, not a good idea. You would likely get it all done, only to find the cab would not fit on around the engine. It is best to have the cab in place, and set the engine to the frame. If you were doing an Explore 302 swap, sure, but with a 4.6 or 5.4, not recommended for you plan.