You don't really have to worry about stagnation with a drainback system. Fluid will not be in the collectors when the system is not on. Slope in a drainback is to ensure water can easily flow back to the tank so it is not left in the collectors to freeze.
I had a very similar situation with a long pipe run and antagonized on what slope I could get away with. It is commonly recommended to have a minimum of 1/4" drop per foot of run for drain back systems. At 40' , you'd only require 10" of drop to adhere to that.
There are a few sources that indicate you can get away with less. Dr. Ben suggests a slope of 1" over 20'
Solar Water Heater Design (9 minutes into video 1.1).
This source even mentions completely level collector installations:
Plumbing Engineer - Features: March 2011: Drainback Solar Hot Water Collection Innovations (see last paragraph). I would be careful with that though as construction tolerances could put you on the wrong side of level.