Good job on the conversion!
I noticed you were thinking about using the quark 80b on 6s but I hear that they have a hard time running on 6s. If anyone else uses a quark 80b on 6s then I too would like to know.
So just to be safe I would recommend only going up to 5s on the quark.
I run a quark 65 on 6s lipo in a hyper8 buggy, and it does fine. Gearing is important when running electronics at the rated limit. If you gear for 40mph you should be fine with the 80b on 6s.