Compatible Parts
 

Will a steering column and steering gear from a 77 f100 fit in my 73 f100?

Need more info to answer your question. Are both power steering? Are they both auto? Two wheel or four wheel drive? It all makes a difference, especially if you have a 4x4.

Voca is correct. As long as you're comparing apples to apples, they will interchange.
2WD Power steering columns are a couple of inches shorter than manual steering.
4WD is much longer, no difference between power and non power because 4WD uses a power assist rather than integral power steering.
One other thing to look for on a '73. Early '73s (before, I believe, s/n S60,001) the plate that seals the column to the firewall is different than later ones and needs to be swapped to make it fit.
As for the steering box, 73-77 will interchange without problems as long as it's the same type.