I assume from your post that the holes in the new mount do not line up with the old ones. I would patch the old holes from the outside. Personally, I would use a dowel and CA. A mix of epoxy and microballons will also work and not run down the inside.

Then drill the new holes. Get some cable like the stuff Sullivan uses for pushrods. Put it through the hole. Get a small 1/16 wheel collar. Put the blind nut on, then the wheel collar. Pull it up tight and the blind nuts should stick. Undo the wheel collar and pull the wire out. Use a bolt with a washer to pull the blind nut the rest of the way into place. Pull the bolt out and install the new mount.

No hatch is needed if you have access through the tank area.

JR