To successfully use a Megasquirt you must use Tuner Studio to calibrate all your sensors. For a Rover V8 4.6L P38 the Coolant Temperature Sensor CTS has detailed calibration information available. Although the default Saab(Bosch) values are a great start.
The standard 4.6 Litre Rover V8 has a really long Serpentine belt. It drives a lot of equipment I don’t need such as air conditioning and power steering. Mine was fitted with a 7PK2320 belt. That’s a 7 ridge flat belt, 2320mm long. With all the ancillaries fitted, the engine simply didn’t come close to fitting in the engine bay.