If you visit the following link ...
http://makelure.com/HowTos.cfm and then watch the 4th video down, you will see how I did it on a bull dawg style bait. Basically you have to taper the body of the tube down to the same height and width as the tail/s coming into the mold. I would suggest making the inserted tail taper much farther down and longer so that will lay inside the body and be encapsulated when pouring the body of the grub. Then the tail will not only be touching but the tail will be interlocked with the tail. The more tail you have laying into the body mold, the more it will fuse and lock together. Hope that makes sense.
Mike