Remember the Mossberg receiver is Alloy( aluminum), this material starts to become soft and plastic at about 800 degrees. I would not heat it anymore than 350F, you can use a welders temp stick that melts at 350 to check this. I've removed these before and I made a wrench similar to a barrel wrench/ vise to encircle the whole tube. This will support the tube equal on all sides so that you do not crush it. A strap wrench may crush the tube in the area where the handle of the wrench is applying pressure. Gunnut69 is correct about blocking the receiver to prevent crushing it. Use a heat gun it will give you enough heat to get the job done, just do not over heat, the loctite will give up easy.
Rebel49