My 86 Mustang has had this annoying rattle in the back, and it seemed to have something to do with the fuel tank. So, I dropped the tank out and found the source of the rattle. There is a baffle inside that is intended to make sure there is always fuel around the in-tank pump. It has dislodged. I can see where welds have broken inside. Is there a way to lock the baffle down again, or is it time to go for a replacement tank? Thanks. - Len
Replace. There should be plenty in salvage yards, especially if you find one that doesn’t drill a hole in the tanks and oil pans.
New Aftermarket Tanks Are Often Not Very Expensive, Also.
I believe you can buy one at a local Auto Parts store for $100 to $150 and start out clean and fresh with usually a couple year warranty.
Yeah, I agree, new tanks are very cheap these days. The baffle would need to be spot welded and would be pretty hard to do that now. Plus no one in their right mind would weld on a tank. I’ve heard of people pumping carbon monoxide in the tank before welding but no good way to get in there to weld it even if you could find someone willing to try. Just buy a new one.
You’ve proven you have the skill and patience to remove the tank. If you’re also extremely frugal and not afraid to drop it again in the unlikely event this does not work, I’ve got a suggestion for you- http://jbweld.net/products/jbweld.php
You can buy this stuff almost anywhere and I’ve used it in gasoline immersion situations before. Clean the surfaces well before application. Working through the tank opening shouldn’t be too difficult if you’re versatile 
You can always remove the baffle. You may need to cut it up with a pair of tin snips to get it out through the sending unit hole.
I run my 86 K5 w/o a baffle and haven’t noticed any difference.