I would agree with tony on this one but with a small alteration. Stick the empeg in the hole where the kenwood is just now but since you are going to be using the head unit as an amp don't cut anything in the dash yet. Found out a way to mount the kenwood somewhere , anywhere in a quick and dirty way duct tape being a favorite method.

Run like this for a while and then decide what you want to do. Be it use the kenwood as an amp or fit a dedicated amp.

If you want to mount a unit in the bottom if it is a reasonably solid plastic you should be able to bend out tabs to hold it in place and maybe something at the back to stop any wobble.