High Level Process, Calculating Cepstrum When You Only Have Spectrum
The cepstrum is defined as:

$C = | \mathcal{F} \log( P(f) ) |^2$

$\mathcal{F}$ is a Fourier transform and $P(f)$ is the power spectrum.  If you've got the power spectrum for the signal there is no need to go back to the time domain.  Just take the logarithm, compute an FFT, take the magnitude squared of the result.

Also, the data you list is strictly positive and real so I'm assuming it's the spectrum.  At the end of your question you put spectrum (FFT).  If what you had was an FFT of a real-valued time domain signal, and not a spectrum, I would expect it to have an imaginary part.