Special Tool for Spark Plug Wires
Slick has a tool for installing the threaded connectors for the plugs.
I made a couple of them a few months ago - one for a friend and one for me. My friend makes up harnesses for a new 2-cylinder Continental type motor. He tells me the tool works just fine.
The tool isn't very hard to make - if you have a bit of machining & welding experience.
If it turns out you need one, I'll be glad to loan you mine for shipping charges only. And while you have it, you can make copies and become the European distributor!
Let me know.