ORIGINAL: RCKen
Just drill out the new holes according to the template. However, I would plug the old holes just to help keep oil and grease out of the tank compartment. Epoxy wooden dowels into the old holes and seal the exposed wood with epoxy and you're good to go.
Ken
Yep he's right... then if you want, sand the dowels flush with the firewall and re-fuel proof it all. I've screwed up drilling the firewall for my engine mounts a few times and what Ken said is what I did and it works great.