I installed the HWH in the original location and ran the engine for an hour under load.
I moved the HWH to a location below the engine and ran the same test.
With HWH above engine the temp was 20-30 degrees higher than when the HWH was below.
I tried to fill the entire system with it up, using a separate line and a valve but it didn't
work very well and yes, you can't remove the cap without making a mess. Now the engine running cooler and I can fill using the cap as designed. No valves.
The one other thing I did is install an inline temp sender at the HWH outlet.