The small load module (silver one) fits just above the ECU barely. I put the big module just... in the carpet on the passengers side right next to the ecu. It's not perfect but I don't feel like searching around. I'm sure there's better places. Good luck, you can do it!!! Just go very slow, very careful, and make sure you double check on every wire that its correct and very TIGHT. It's doable.
Umm I'm not positive on fuel economy. I think if I didn't drive crazy every once in a while it would be better... you have to realize, with the module you don't take it up in the high RPMS and thats where you eat your fuel... I think if I drove like I used to without the module, It would be better haha!
And don't worry, Richie will get you it... it took me a few months to get mine too. That's why I asked ahead of time lol.