1961 height as back surround?

Thank you for that, now I understand dimensions better.
I personally would not bother with back surrounds in that case, because it will stretch out the sound bubble more to the back and not enough on your listening space.
Transitions from the side to the back surrounds will not be coherent and sound much different, as the sound has to travel much longer distances.

But in the end, it is all personal preference. Maybe you can try this first with others speakers you have?