I'll do you one better...
In 1958, in conjunction with the release of his book Eight Hours Before Richmond, author Virgil Carrington Jones hired a police handwriting expert named Ira Gullickson to examine the photos of the Dahlgren papers and to write a report of his analysis of their authenticity. I happen to own one of two known copies of the report--I happened to purchase Jones' own copy of his book, and the report was tucked inside the book when I bought it. Fortunately, Admiral Dahlgren was quite the pack rat and saved every scrap of paper, and when he died, his widow donated about 10,000 pages to the Library of Congress, including Ully's diary and every letter that Ully ever sent to his father, so there are plenty of exemplars to choose from.
Using what was the state of the art in handwriting analysis in 1958, Gullickson concluded that the documents are authentic. I agree completely. Having read all of that correspondence and the diary in the course of writing my biography of Dahlgren, I am quite familiar with Ully Dahlgren's handwriting, and there's little doubt in my mind that they were written entirely in his very neat, very precise handwriting. I say so in an appendix to my book, and I quote from the report at some length in that appendix.
The bigger question is not the authenticity of the documents, but rather who was involved: was Dahlgren cowboying? Or was this part of a bigger plot? After years of researching this, I came to the conclusion that Stanton and Kilpatrick cooked up the plot, found a very willing participant in Dahlgren--he had proposed a raid on Richmond in the spring of 1863 that was rejected by Stanton--and then disavowed him after the raid failed. Again, when you know this story as well as I do, it's quite clear that Kilpatrick lied through his teeth in disavowing Dahlgren.
The proof is in the pudding. Not long after Richmond fell, Stanton ordered that the originals of the Dahlgren papers be brought to him. Francis Lieber delivered them to Stanton, and they were never seen again. Conclusion: Stanton burned them in the hope of covering this up. Fortunately for us modern historians, there were several photographic copies made of them that survived the war; I know of two sets of original photographic copies, both safely in archives.