Don't forget about the Lower back<<Essential & Obliques (Core). The stronger your lower back, the easier your ab workouts will become. Trust me.
Ab workouts also hit the Hip Flexors which are an essential muscle group in turning them cranks.
All I will say is... Armstrong, Ullrich, Cancellera & Hoy all lift weights, all got too the top of there sport, beating everyone under the sun that don't lift. Coincidence??... Or just "stronger" rider beats weaker???...