Not sure what the size is on a Sun machine but you can buy about any square size stock at any decent hardware store. And a wooden handle of some sort and that’s your tool to snug down the distributor.
Also, I’m not sure why the 6 bolt battery. If you can use a 12 volt I would. That’s what I use.
Also not sure why you can’t use the battery to run a aftermarket tach so you can test higher than 5000 RPM. You’d be surprised what you’ll find. Also, if you do the 12 volt battery, you can hook up any ignition system and bench test it.
Unless the Sun is that much different than the Allen.