What Steve said. But it can also be a regular part-discrepancy issue as well.
Many years ago I purchased a set of Moog tie-rods and had to put a washer under the nut too. It wasn't needing much, but it was not a perfect fit like the original either. And that was with stock '71 D44 knuckles.
But if yours is way off, it might be just as Steve mentioned as having had some sort of a parts swap done in the past.
Do you know the history of the Bronco? Or do you just know that they're stock knuckles?
Bottom line though, is if the taper is the same and you get a good tight fit with just a regular or slightly thick washer, you're safe to do so as far as I'm concerned.
I used grade-8 washers anyway, because I don't like things like steering components loosening up over time(!) because something like a cheap a washer was somehow distorting. Of course...
Paul