A diabatic correlation diagram technique is extended to assign effective quantum numbers and classify sequences for extremely high excitations in a coupled two-mode model of an isomerizing system, with multiple wells separated by a potential barrier. At low values of the stretch quantum number n s , level spacings for sequences of bend excitations n b = 0, . . . show a pattern of a smooth dip at the barrier, characteristic of the zero-order uncoupled system. In higher sequences n s = 3 − 5, the spectral pattern is modified with the onset of a prominent nonlinear resonance. The level spacing "tunes" to a flattened pattern similar to a harmonic oscillator, and the smooth dip at the barrier becomes almost vertical. This behavior is explained by the influence of periodic orbits of the resonance on the quantum spectrum and wave functions. In the n s = 6 sequence the tuning reverts to a pattern more similar to zero order.