you should be able to put the tubing overtop of the threading on the outside of the bulkhead. If it's too tight, you can try to but the end of the tube in some boiling water to soften it up so you can stretch it out. If this works, you'll need to get one of those metal or plastic ring clamps in the plumping department at Home Depot or Lowes so that you can tighten the it.
Another thing you can try is to take the overflow to box to HD or Lowes, and get a threaded coupler 1.5 size pvc... or reducer from 1.5 to a 1 1/4 inch... get a small piece of 1 1/4 pvc pipe about 2 inches long.. and that should fit... then put the tube over top of the pvc pipe and use the clamp to tighten.