I think the box you are talking about is the Quadzilla XZT tow module. It is good for 65hp and while it does need to be mounted in the cab, it is non-adjustable so it can go under the dash.
That is exactly the one I first considered, and you can find them on ebay for more like $250. I ended up going with the Quadzilla standard box with a switch for 30,60,100 hp for about $340, also a very good buy. I normally run on the 60 hp setting anyway so the XZT would still have been a very good choice for me.
You will need to have an EGT gauge in your truck though, no matter which box/programmer you choose, I don't care if the manufacturers say you dont. Any fueling box can and likely will significantly raise EGTs and you need to watch them.
That said, the power is awesome, very noticable over stock. This was my first power upgrade on a CTD and I am hooked. I would also very seriously consider and air intake horn from somebody. I bought the CFM+ and it made a huge difference in lower EGTs and quicker spool up of the turbo. I would without a doubt buy the air horn first over any of the air filter/airbox mods, I think it makes a much bigger improvement in airflow.