This is wrong.
In the frame of the "moving" observer, the line can have arbitrary length d' and velocity v', such that d'/v'=t'=57.74 seconds (which is all that you stipulated). For instance, if d'=1000cm and v'=17.32c then t'=57.74s. Or if d'=10cm and v'=0.1732c then t'=57.74s. So both length contraction and length dilation is consistent with the line passing the observer in 57.74s on his clock. Hence d'=100cm does not follow from t'=57.74s.
This, of course, is really fucking obvious, because we have 1 linear equation (d'=v' * 57.74s) with 2 unknowns d' and v', which has infinitely many solutions. Or to use an analogy, if someone tells you that I can do a lap on a racetrack in 57.74s, that in itself does not (and cannot) tell you how long each lap is.
There isn't a unique solution to d'=v't' unless you know values for two of the variables. That's just the way it is. You cannot, not even in principle, prove length invariant from time dilation alone. It's impossible.
But you're welcome to try some other way, of course.