If you've got the 60/40 bench, you'll need to measure the 60 portion from lower mounting bracket across to the other. Then see if your Zapco is less than that measurement. If it is, you can make it fit by remounting the amp over to that side. If it is longer than that, it wont fit back there easily. From that picture, I'd guess it will, but that doesn't look like a Ram's back seat. Here's a few pictures of mine. My amp's length is 25". If that's a Zapco DC1000.4, I think it lists the length at 24.4" so you'll be fine.
You'll need to arrange it between the two seat brackets, so you probably can't just slap that same board back there and have it work. Your seats wont be able to go all the way up, but as long as you mind the metal brackets, they wont damage your amp.
I've got some more pictures of little things, and I'd be happy to go into detail on the build. I had to use trial and error quite a few times with mine and wasted 2 full sheets of MDF. If you get it right the first time, it's not so bad.
Zapco's aren't made for big boom, but the flexibility.....