Many systems have taken the vertical mount approach approach over the years: Blade servers, routers, modems, and various gaming systems.
Horizontally-mounted: parallel with the floor
Vertically-mounted: perpendicular to the floor
When there are no fans, there are some empirical rules that give an approximation of the heat flow, but are applicable to very specific cases.
If there is a cooler and a fan inside the racks, my guess is that nobody know.
Note that a big fan on the side may also help to change the air between the racks faster and increase the heat flow. Some motherboards have a fan that sucks fresh air from outside the case and send it directly to the cooler.
Probably the only way to know is to try it. If you have a few spare boards you can try the different configurations. if you measure the temperature in each case and take a few photos, it can be an interesting blog post.
Are engine cylinders ever mounted horizontally? Why or why not?