Hi, Engineer here so 2 cents.
The problem you will face is that when you suck air in from the bottom like that, is the majority will go the bottom racks, and the top racks will not get as much cool air, or if they do the air they get will be hotter.
A few things you could try:
Make each stack its own unit, have cold air come into the bottom for rack 1, then have a separate area for the air to come into the rack number 2 up top.
Also you could facilitate VELOCITY to cool the GPUs. If you have a smaller intake than you do exhaust then by the laws of fluid mechanics ( like how air moves also) the VELOCITY of the smaller intake will have to increase in order to move the same volume through the piping. Think of a fan on a hot day, do you feel cooler with faster air hitting your skin, or with slow air? Same concept.
Hope I helped