Darren,
It isn't just the threads, it is all of it. Because it isn't a solid lump, it will be nowhere near as strong as if it was all one solid spindle. That is why I kept mine down to the smaller sizes and do no heavy cutting with them.
I saw that die when you showed it earlier. Really, what you should do is single point the thread roughly to size, then finish off with the die to give the true thread form. If you try to cut in one go, just using the die, you only have to be a gnats out of square and you will be in deep doodoo.
Really, the best bet would be to mount the blank into your 5C carrier, then cut the thread using single point. But rather than Myford, I would use something like a Boxford thread, which is the same as Atlas, some South Bend and a few other large lathes. That would then give you access to much cheaper second hand nose fittings, rather than Myford, which tend to be expensive even second hand, purely because of the name.
John