[Mhu]

Even if you're using heavy duty wrenches, the corrosion in the threads (which technically weld the pieces together) may not allow you to remove the bolts without breakage. Try a GOOD penetrating oil (scrap the WD40 and try somethig like 3-in-1) and soak the parts. Let 'em set for a couple of hours then soak 'em again. Give it a day or so to work before taking the wrenches to it. Heating the connection may work, but I'd rely on that only as a last resort.