Earlier published theoretical models for MeV electrons are generalized in this paper. Different theoretical predictions for planar channeling and accompanying electromagnetic radiation in thick crystals are presented. A comparative analysis of theoretical and experimental spectra of photon radiation is given. Thickness dependences of channeling quantum state populations, radiation line broadening and photon flux intensities are obtained. The existence of planar electron channeling at 54 MeV in a silicon single crystal with a thickness of several millimetres is shown.