I had the same problem with my Edelbrock heads. I tried different pipe fittings and nipples - same problem. You are likely going to have to run a pipe tap slightly deeper in the head to get more engagement with the sender(s).
I assume that the heads are already installed. If so, push a small rag into the hole, apply grease on the tap and turn it in a half turn after it starts to cut. Remove the tap. Most of the shavings with stay on the grease, but some will fall in the hole, hopefully most of the shavings will be captured on the rag. Use a thin pair of needle nose pliers and pull up on the rag and use air to blow chips up off the rag and out of the hole. You will likely lose a few chips down in the water jackets, but not many if you are careful.
Try the sender again. You will probably have to run the tap in the hole again, but this is better than trying to go too far at one time.
Not the best solution, but it beats removing a head(s).
I'm sure that even experienced engine builders learned the hard way to check sender, pipe plug and even spark plug engagement before installing the heads.
Good Luck!