Steering shaft nut / threads issues

The steering wheel on my Barracida was just stuck on the shaft (barely) with no nut.....so I started looking into gettin ga nut...

Someone on the cuda-challenger.com site said it is is a 1/2" fine lock nut.

Well,I found a couple of 1/2-fine (20) nuts.....one a stop nut the other a nylon lock nut.

However, seems that the top of the shaft is a little mushroomed over.....I have a thread chaser die and a thread file and thought I was making a little progress but no joy.....the die doesnt seem to want to start ......

Afraid to get too aggressive and really louse up the shaft.....

Any words of wisdom on this?