If you leave it an inch from the top and use a recovery tank the level in the rad will slowly go down and eventually you'll start running hot.
Put enough fluid in the recovery tank to cover the hose end by 2 inches, fill the rad to the top and run it. After the first cool down check and maintain the level in the tank.
Or just set it up the old way a couple inches down and use the tank as a catch can.
How do the tanks on your original rad like the pressure? THey weren't made for even 7 lbs.