There is a good bit of room there for amps once you cut the studs out. More on the right than left because of wiring for the pump.
The one you can see in the pic is a Pioneer 700W amp for the sub. It is more than double the length of the stocker and several inches wider. So it sticks out into the trunk area a little bit. I am cutting off the studs that the stockers mounted too so the new amps will sit flat. What you dont see is 2 more Sony 250w 2 channel amps. 1 in the stock location on the left side (runs the front door speakers and tweeter) and one more directly behind the vert pump (for the rear speakers and tweeters). If I were to change anything I would put a single 4 channel amp on the left where the stocker went. This has got to be the easiest stereo install Ive done. With the seats out of the way you have access to everything.
My car had the Mach at one time. But when I got it the head units were gone and a POS Kennwood deck was cobbled in there. So I cut the POS out and removed all the 460 parts that were left. I am running all new wires for the speakers and power to the amps. But I did make use of the 460 speaker mounts to put all my Pioneer Premier speakers in.
Everything is Pioneer except for the 2 Sony amps (on clearance at Walmart) and the head unit which is also Sony. When you turn the stereo off the front screen is completely blank. You cant even tell there is a stereo there.