Definitely does not sound like a nano to me.
Wow sounds like you have everthing well planned. Moving the critters is gonna be tough no matter what you do. I think I would try to move the 29 with livestock or at least at the same time keeping the substrate undisturbed and keeping all the old water etc. Then basically you have the cube just in a different place temporarily.
Then fill the 75 and after it is done cycling move the livestock slowly to the 75. This way the bioload can be increased gradually giving the bacteria time to catch up. Then when the 29 is empty you can make it whatever you like.
I really am no expert since I have never moved a tank, but I just like the idea of keeping them in their existing home while the new digs are prepared and slowly introducing them to the 75 to reduce spikes.
I assume you are going to seed the 75 with water and some substrate. I would at least. Give things a bit of a kick start on the 75.
Best wishes and keep us posted.
You know we want pics.