Complex sentence: Due to the very different disposition of the wings and fuselage, adaptation of the mounting to the S.E.5a was not altogether straightforward; the well known "S.E.5" version of the mount required a much longer, more robust sliding quadrant rail.
Split into simpler sentences: Due to the different disposition of the wings and fuselage, adaptation of the mounting to the S.E.5a was difficult. The "S.E.5" version of the mount required a much longer and more robust sliding quadrant rail.