As good as JB Weld is...I don't believe it will be a secure method of securing the bolts to attach yer belly pan to. Keep in mind that these bolts will be subjected to more than just vertical forces as the belly pan will exert more forces than up and down.
I would see what available attachment points are available under yer vehicle now. Even using some bracing or brackets you can attach the belly pan to. I ended up using existing fasteners or holes in my truck frame to attach a framework to. To remove my belly pan only requires the removal of a minimal number of fasteners.
If I didn't post it before...here's my thread.
Under belly pan... - Toyota Nation Forum : Toyota Car and Truck Forums