Hi Tonycharger72,
When I was doing that stage, before I removed the L6, I put a rod from the centre of the crank pulley at 90 degrees with a couple of welders magnetic squares and when this is brought forward it comes to the front center lower bolt hole for the grill. I then used a small laser with a packing piece under it to line this up with the crank, and with the engine out it lines up with the center of the diff. So you can measure how far back the back of the gearbox is and then using the laser and a tape measure, take some measurements from certain reference points.
I hope that this makes sense.
Rob