Harry, I have seen a few mention that they used drill rod for their support rods. I have used my lathe to turn a 1/2" drill bit shank down to 3/8" so I could chuck it in a 3/8 drill but I haven't tried threading one with a die. Have you?
A split die is theoretically adjustable. There is a recessed hole at 90 deg to the split so that if you tighten down the grub screw that keeps the die from turning in the holder you would also reduce the % of threadform. A common bolt has a threadform of about 70-75%. At least that's the way that I understand it. Maybe Harry could verify that. I'm sure he has more experience on the subject than I do.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.