The mono bit sticks out like a very sore thumb.
I think smoothing the transition to/from mono helps a lot. I usually crossfade from stereo to mono and back, usually ~5-15 seconds on either end of the mono section. Makes the change much less abrupt and easier on the ears.
Edit to add: if you want to deal with individual tracks, you can also split the stereo file into mono files:
File | Export Audio | Mono / Stereo Convert | Stereo-Wave -> 2 Mono. This will allow you to treat L and R channel objects independently. Note this will create new mono WAVs on your HDD.