I think you are looking for a mountain board with those wheels. You’ll get terrible wheel bite even if you can mount the wheels without the longboard deck getting in the way.
Good luck with those. 5 years after those break down, I’ll still be riding my Dark Matter motors. Did you notice that of all the esk8 motors you can buy, out of every single motor on the ENTIRE market, only one shows you photos of the inside of the motor? Do you know why?
Those hubs are somewhat crappy.
Out of balance with pressed in bearings. You’ll have a good time trying to replace them once they seize up (and they will).
I don’t know how serious you are about offroading, but for serious mud splattering offroad i wouldn’t suggest a regular longboard with big wheels. You would have to get a proper mountainboard like mbs or trampa with bindings and all. Also a Hobbyking VESC wouldn’t be enough for this application, use at least a focbox similar that is more robust.
For a longboard hybrid build you should maybe look a 6" wheels from psychotiller or whoever also sells them. They are better for regular trucks and more street oriented decks.
As you said here, for not so serious offroad like occasional driving over grass/dirt/snow 6" should be enough. There are also those 6" airless tires from diyeboard.com but i can’t tell you if they are any good.