(1) The requirements of ARM 17.8.402 do not apply to stack heights in existence or dispersion techniques implemented on or before December 31, 1970, except when pollutants are being emitted from such stacks or using such dispersion techniques by stationary sources (as defined by ARM 17.8.801(28)) that were constructed or reconstructed or for which major modifications (as defined in ARM 17.8.801(20)) were carried out after December 31, 1970.