What differences are there between a '77 and a '77.5 F250 4x4?

Do factory front high boy springs fit a '77.5 F250? Would they lift the truck much?

 

Differences, '77 is a High Boy the '77.5 is a Low Boy. High Boy has low pinion front axle, gas tank in cab, narrow rear frame rails, divorced transfer case, 2 1/2" rear springs, pos. arch front springs, etc.
Yes, front springs from the High Boy will fit the low boy. A good stock set might get you around 3"

 

Does the highboy use the same steering as mine, or the power assist manual steering?

 

highboys have power assist steering

 

Manual steering box with power assist, which works pretty well when in working order but can be very pricey to replace.