The 3C-6H polytypic transition in 3C-SiC single crystals is studied by means of diffuse X-ray scattering (DXS) coupled with numerical simulations. It is shown that the presence of spatially correlated stacking faults (characteristic of this type of re-stacking transition) gives rise to extended diffuse scattering in the reciprocal space perpendicularly to the fault plane. The simulation of the diffuse intensity allows to determine both the volume fraction of transformed material and the transformation level within these regions. It is further shown that the evolution with time and temperature of the transition implies the multiplication and glide of partial dislocations, the kinetics of which are quantified by means of DXS.