In a closed loop, the height difference does not matter.
If the coolant goes from the rack down to the cooler and back up to the rack, then gravity will help with leveling the coolant, reducing the effective height difference to zero, Any D5 can handle this.
The problem is filling the loop, when the pump has to fight against gravity in a loop partially filled with air.
Once the air is gone and all the hoses are filled with water, the D5 pump will work just fine.
Filling shoudn't be a problem if you strategically place fill and vent points at the very top of the loop (maybe have the resevoir at top as well) and have both open when filling.
That way the pump isnt doing the work