A good design shouldn't need to use thick plate or other heavy sections. You'll never see much thicker than 3mm plate on a factory car, even a new 4wd ute. The original subframe uses a mix of 2.5 and 2mm sections (or their imperial equivalent).
The trouble with thick steel sections is they are too stiff for what they are attached to and generally have low moments for their mass.
Here are some pics of NG's subframe from a few years ago.
Gearbox side. See how the original section is replaced with a similar Z section that goes around the gearbox and holds up the engine mount. Also note it is continuous under the drive shaft hole all the way back.
Another view of gearbox side
Here is the engine side. Again the engine is held up by a fabricated section.
Another view of engine side. It would have been possible to make this section deeper back from where the track rod passes under. You can't see in this pic but the bulge shape is to clear the crank pulley.
So the above example uses three engine mounts. One under the gearbox and one either side of the engine. There is also a supplimentary torque reaction mount up near the master cylinders but it doesn't add much. I would be tempted to not use the mounts on the side of the engine and have a single on the timing belt side plus a fabricated structure for torque reaction reaching back down the tunnel. That would take the load off the cross element at the front of the subframe.
Some more photos in that photobucket album.
M