Best position for the battery is the shortest cable run to the starter motor, so preferably in the middle of the bulkhead, where your heater was.
Best position for the ECU, and to make a tidy job is looking at the photo, turn the ECU around 180 degrees so the multi plug is facing the centre of the car.
Regarding throttle cable, if you can obtain cable that is appears to be too long, run it forward under the throttle bodies, do a big loop and come up under the throttle bodies, so you have a smooth long curve in the cable.
This may sound wrong but it should work out OK.