It happends that freehub bodies are not exactly fitting when interchanging them. You sometimes have to include a thicker shim in between the hub-shell and the free-hub body.
Once the tightening is done firmly with the 10 mm allen key, make sure the freehub body rotates smoothly.
And always use the appropriate cones and nuts belonging to the freehub body in order to respect the proper OLD alignment.
My road winter wheels are using 6600 UltÃ©gra hubs with 7700 titanium freewheel bodies. They are theoritically incompatible with each other, but with using the cones from 7700 within the freewheel side, and the 6600 cones on the other side, I have 20.000 + Km with the wheels.
The best of both worlds.