My only concern is that the center bolt is only holding the left side in place. There is going to be a ton of rotational torque working against the layshaft and that center bolt. I can see it twisting and splitting the aluminum tranny case with the power of your 70cc. With a smaller motor, I think the rest of the mounts will keep her together. I might also be underestimating the over engineering HPI put into that tranny and the top plate. But with that big sumbitch of a motor you're building, there's no way to tell until you get it put together, and let 'er rip.
I say run the poop out of it. If it explodes, try again!