You forgot to mention the type of CPU that your running. Different CPU's have different operating and danger temperatures.
I'd say the best thing that you can do is have a nice amount of air flow going through the box to keep the system cool. Being inside the cabin means that its going to be air con'ed in summer (presuming that you have air conditioning). But having it on the floor its going to pick up some heat as the floor of a car gets hot when driving around in summer.