You're right. I think the ATC450 could also be swapped in or it looks like it may be possible to put a spacer between the 8HP and the ATC700. In the pictures, the ATC700 has an almost flush mounting flange whereas the ATC450 is recessed. I also did some more research and found the diffs are different. Early models received very short gears - 4.44 in the gas 3.0 and 3.91 in the 4.8. Later models received 3.15s which works with the broader torque curve of the turbo engines & the wider ratio spread of the 8HP. While the 8th gear is about comparable to 6th in the 6HP, first gear is much shorter on the 8HP. I think I'd prefer to try and keep the 3.91s if possible.

I also looked up the immobilizer and it looks like the CAS3 was used for the entire production run of the E70. That means it should absolutely work with the 8HP.