Yeah, good solution is to put the motherboards and their guts vertically with, if possible in one horizontal line with corresponding air ducts above them to direct the flow through duct with bigger inner diameter + good industrial/home appliance fan at the end. Putting small 140mm 2BB fans below the cards will ensure almost proper push-pull design. One of the major cons is that the fluid/el bearing fans of the cards/cpus will suffer faster from scratchy sounds - their axles can't sustain very well the horizontal position.