I don't doubt that Dr. Fujita rarely went into the field.
But I find it interesting that there was, as you say, evidence of a tornado of that size (twice the current record that was, yes, in Oklahoma a handful of years ago), but there is no official evidence of a tornado of any size 20 miles west of Grand Island during this outbreak.
The Wikipedia article for the Grand Island tornado outbreak mentions a twister as far away as West Virginia, but there is nary a mention that I can find of a tornado 20 miles west of Grand Island, regardless of size. How can there be "unofficial" evidence of a tornado being that size if there isn't even confirmed evidence that there was a tornado at all?
I'm struggling to grasp the concept of there being evidence of a tornado five miles wide when no one (Wikipedia or NWS) mentions even the possibility of there having been a tornado there? One would think that the possibility of having a twister of that magnitude (official or not) would be something worth reporting. It's not even mentioned in Wikipedia's page of tornado records which, as you say, has some unofficial tornadoes listed as more than 4 miles wide. Wouldn't the word of Dr. Fujita himself be enough to include this (apparently non-existent) twister to that list?