1972 Aux gas tanks

[JJA1976]

I need a little help with theses saddle tanks. I could not get the bolts to come loose and had to grind the head off of the bolt in the inside of the box. I now have them off and need to get the rest of the bolt out. I was wondering if anyone knew exatcly how these were attached. Mine have the 3/8 drive hole in the bottom and that just twists and don't move. I am wondering if there is a nipple that is welded inside the tube that the bolt goes through or what. Do I need a BFH and just pound them out? If somone has any experience with these and idea's would be greatly appreciated.

Thanks,
J.J.