What to do with my gas tank?

You should be able to remove the original and reglue it to your new one. Take your time with a putty knife and peel it off.

I tried, it would not come off. I was hoping it would, then use spray adhesive.

A piece of heavy felt paper will work. You will have to slice it where it drops down around the tire well to conform.

Do you think like a craft store? I have never seen felt paper anywhere.I guess if I put it up there metal to metal, thats a bad thing...right?