So you're saying with his current Mobo and CPU cooler arrangement, he can't find a case that is larger, and will provide better airflow for his system?
( Also, you said CPU instead of PSU
no mate, it's impossible... i had a Dell machine and i know how their design works... simply he is going to do a hard modd process and it won't work perfectly....
the problem is not the larger case the problem is that Dell measures every inch in their PCBs and accordingly they design the case... it looks like this
and sorry for the PSU mistake