I mentioned in my disc brake swap that I had already converted my spindle nuts to a more modern setup.  The stock castle nut preload method is a little too loosey goosey for my liking and not good at all on a disc brake swap because any wobble in the rotor will force the pads away from the rotor and cause a delay in brake application.     I used Dorman 04991, which is a 3/4-16 nut, .34" thick.  Ford truck applications. Dorman 04990 will work as well, it is slightly thicker at .37"