It fits!

With the driveshafts fitted (look closely under the motor and on either side of the subframe) it seems reasonably clear that the gearbox & motor will fit into the subframe. The motor isn't quite round and is very close to the suspension tower and the flat surface of the subframe. This means the motor has to be rotated correctly to avoid hitting the frame.

I'm tempted to get the shaft ground, assemble the gearbox, slide the motor onto the shaft and then build the adapter plate around the pair while they are in the subframe. It wouldn't be fun get the adapter plate back and find that the motor needs to be rotated a few degrees.

Next task is to strip the input shaft (it's covered in overdrive mechanism) ready for resplining to fit the motor.