77/78 K tubes will fit, and the whole leg assembly..lower are longer, and easier to get fork braces for.
77/78 F forks have a diff lower and brakes, tubes may work, but I am not sure on them..
I had a problem with mixing and matching tubes/lowers. I am NOT sure of the years, but in addition to longer lowers, the damping rods and the corresponding bottom out cones are different between the years and you have to be careful when mixing those. Early years centered the bottom out cone in the lower by a the cone having a 35mm "foot", later cones did not have a wide foot, but fit in a pocket machined in the lower.....those probably should not be mixed
The damping rods also differ between the years in their length, diameter of the shaft, damping holes, the piston/roll pin arrangent.
The bottom line is: the diameter of the damping rod shaft needs to be the same as that of the (new) tube and that may necessitate the use of different damping rods, which may necessitate a look at the bottom out cones. Do not use the (smaller) cones that are designed to be centered by a machined pocket in the lower that does not have such pocket, relying on the bottom bolt to center it!
It is pretty simple when you see it, just my retarded explanation seems complicated...
--Nick