After having the gearbox out a couple of times the idea of having only lower studs is interesting. Re the strength issue allot of the stress would be rotational torque becoming shear on the remaining bolts and potentially shear on the 2 lower studs. While the studs may be not as effective as a machine bolt secured on the inside it still may give some structural effectiveness.
This change would allow you to slide the box onto these two lower studs and may make for an easier clutch spine shaft alignment than the full gearbox / bell housing assembly.
If you did it please note that the clearance to the face of the pressure plate rotating assembly from the face of the inside surface of the bell housing will likely be less than 6mm. The studs therefore should only be slightly longer in depth than the bell housing depth.
Otherwise you could catch the clutch rotating assembly.
Which is bad.
Happened to a friend of mine .....!