Agree with Lock n' Load.
There is no need to provide lateral saparation for wake purposes bewteen aircraft that are separated vertically by 1000ft even though wake sinks.
Provided the 3nm radar separation is maintained, the B767 can be descended as soon as it has cleared the path of the C402 because when the wake is generated at [A] in the diagram, the vortex are 1000ft above the C402. During the time it takes the C402 to reach [A], the wake will - sink, diverge from the track of the B767 (depending on wind) and partially disipate. Provided the C402 does not track east of the cutrrent track, it makes no difference what wake exists to that side of track.
Regards,
DFC