The fan should face down. It is blowing cooler air from down to up, into the PSU where it flows out the back. If you mount it so that the fan is up your are fighting the laws of nature. Heat rises.
In reality install the PSU any way it will fit. Upside down or side ways wont have any impact on the operating temp in your system.
I use a dremel to mod HP and dell casses to take standard PSUs all the time. They always seem to have a little bit of mesh just in the way for the power plug or the on off switch.