I also have a Gano filter in my upper radiator hose, I can visually see what crud is passing through the coolant system.
I do not have any advice for a coolant flush product, aside from a garden hose with the engine running for a while.
Burp the system.
Did you clean out the coolant passages when you rebuilt the engine? There is a lot of crud that builds up on the backside of the #6 cylinder. If that stuff started migrating through the system it could clog up the top of the radiator. I learned the hard way.