I would just bring it up to cherry color.  Get my torch shut off and laid aside.  Get the 4 x 6 in position.  Weld it up all around 360 degrees.  Relight the torch and keep it warm so it does not cool off to fast.  Gradually removing heat.   

Are you grinding back the edge of the sq. tube before you weld?  A true good weld should penetrate all the way to the inside of the sq. tube.  This can be accomplished by the grinding back and/or by have a washer under the sq. tube to give you a 1/16 to 1/8 inch space between the sq. tube and the flat iron. 

You will need a slightly lower amp setting on the welder because of the hotter flat iron.  I know when I preheat cast before I weld I need to turn the welder down any where from 20 to 40 amps or else I burn all the metal away. 

I do not know of any mfg. that goes to all the trouble I have written above.  You must be putting a great deal of strain on the 4 x 6 to require such a weld. Yes ?