Quote:

On all my Mopars, I have placed the ECU under the battery tray.



I did the same on my challenger too. This way no holes are drilled in the body, (only in the battery tray), it is in a place that is nice and cool, and fairly well protected. It is also totally invisible.

Lengthening the wires is not a problem at all, in fact it was stated in the instructions that came with my kit.
I had zero problems with this, has been on the car for 10 years.