2511 was set up by Dennis Quella for a shoulder harness with a single attachment point. He added a vertical U-channel to the steel firewall which included the new anchor point.
I found that anchor point to be too low, so it did not properly thread through the harness hole in the top of my Recaro seats.
Using 3/16” steel plate, tubing and a nut, a new anchor point was fabricated. A clearance hole was then drilled in the top beam of the firewall and the plate welded in place.
If you have the ability or access to the welding skills for this approach, I believe it would serve your purposes quite satisfactorily and be much easier and more permanent then fighting with a gas tank and using JB Weld.
Larry