Installed the Harbor Freight clamp today. I choked the line off completely, set the HVAC to hot, and waited for the temp to stabilize. Then I opened the clamp little by little until the temp started to rise a few degrees. Left it there so I have at least a little flow through the system. Stagnant water in the heater core and lines sounds like a terrible idea for a long period of time. Maybe the antifreeze would prevent any growth, but the heat certainly plays a roll in killing everything that would grow in there too.
At first my AC vent still showed mid 50s out the vents... wasn't happy and thought it didn't work for me, or maybe the little bit of flow was too much. Then I thought to try driving around the block. Sure enough, as soon as the truck started moving the temp dropped to 45 degrees. Much better.