RE: 69" QQ Yak-54
If you use 5 cell NIMH I think you should balance fine! It would be really difficult to balance with light weight 2 cell Li-Ion. I think your engine would be better if used in 69" rather than 73". How much vibration does that engine have? Is it like FZ or more like the DZ? If it is a shaker then I would think about soft mount as well.
I think you can do it, go for it! but remember it will require you to do some more work and be a little more patient as you need to start shifting things around to fit your engine and balance.
Talk to you soon,
Reza