It would be best for air to come in from the lower front of the case and to be expelled through the power supply towards the back of the computer.
You could reverse the fan in the PSU but remember tinkering inside a power supply is not recommended as it stores lethal voltages even when turned off.
In workshops the voltage stored in the capacitors is quickly drained using a special tool before commencing work on the unit.
Other than that the best solution for airflow would be to put everything in a tower and use the fans in that.
