Looking at your stand, I'd be concerned that the legs don't extend far enough to the rear. That's a pretty large motor and countershaft assembly, and it looks like it's already well behind the rear feet of the stand ... might try to tip the whole shootin' match over backwards. Are you planning to secure the legs to the floor?
As for supporting the motor ... most of its weight will be supported by the V-belt going to the countershaft. The pivot shaft keeps the motor parallel. It looks like it will be stout enough. This is, after all, how the lathe was set up originally, isn't it?