The main issue I could see with the threads being all the way up is that you may not be able to torque it properly and I know the bolt tapers at the top to provide a snug fit at the top so that could cause issues of some sort too, overall I don't really know the dangers of that so everything I've said just now is conjecture.
The bolt needs to be hardened, this is obviously what connects the steering wheel to the gearbox so you do not want it to fail. There should be markings on the head of your bolt unless they've faded off and that will tell you what grade and material it's made out of. I did a quick google search and came up with
this graph that ought to help you out. I stopped looking for one when I realized I would have to special order one, you may have to do just that. Unless someone smarter than me(and there are lots of those people here
) can tell you it's safe to do otherwise.
EDIT: Let me preface this by saying I am not familiar with the company I shared the graph from, but it looks like if you know the grade of the bolt and the thread pattern you could order it from them.