Right you are sir, sorry I wasn't more clear. A 9/16x18 should be drilled with a 33/64" bit ideally. So you'd pretty much be removing the 1/2" threads with the drill and re-tapping. This assumes the material of the 1/2" piece will be thick enough to safely support the 9/16" bolt and the loads. This seems a little easier than the other options like crafting your own inner tie rods. If you were willing to do that, I have no doubt you can do this successfully. Ideally you could find the right sized female joint, but if not, it seems like a fairly easy-to-try option.