Ok. That's perfect. The only real impediment to the gpu fans is going to be a layer of mosquito screen to make less dust come through. the sides will be blocked by the case i'm building or it. And downwards other than through the psu the air can also go through the hole i've cut to let the riser through, allowing it to go through the empty side of the chassis as well. So it should be pretty mutch a straight pipe flow wise.
I was a bit wary because the only thing i've really built cooling paths for were transformers from welding machines. And those might as well be heaters.
Thanks for the info.