Jens, you're correct about using the front panel 40G QSFP+ ports for stacking on the X670-G2. There is no module available for the back of this switch model.
Here's some additional information, which you may already know, about stacking on the X670-G2.
Regarding licensing, the X670-G2 comes with an Advanced Edge license, which can be upgraded to core. There is no minimum license required for stacking, though all master-capable nodes will need to have the same base and feature licenses.