Need help determining the correct size of hexagonal rod to buy

[Monty]

I'm building an old Ford that will have a wired distributor with vacuum advance.  The v/a module looks like this:

This type of hexagonal rod I need to make this isn't available locally so I'll have to get it off the internet.  I have an idea which size might be the best choice, but I'd like to get your opinions.  

Looks like I can get these in .020, .030, .040 and .060.  

Being hexagonal, the rods can be cut down as bolts for various other applications etc.

[Plowboy]

IIRC, that's 3/4 of an inch. So, it would be .030 in scale.