I checked a reader I have of Che's, but that letter is not included.

However, looking at the article again that quotation isn't from the letter to his aunt and there is no source cited for that quotation:

In 1953, situated in Guatemala, the 25 years old then Che noted in his letter to aunt Beatriz: “Along the way, I had the opportunity to pass through the dominions of the United Fruit, convincing me once again of just how terrible these capitalist octopuses are. I have sworn before a picture of the old and mourned comrade Stalin that I won’t rest until I see these capitalist octopuses annihilated” (Jon Lee Anderson, Che Guevara: A Revolutionary Life, 1997).

Years ago after his letter from Guatemala- in the midst of the revolutionary process in Cuba- Guevara would re-affirm his position towards Stalin:

“In the so called mistakes of Stalin lies the difference between a revolutionary attitude and a revisionist attitude. You have to look at Stalin in the historical context in which he moves, you don’t have to look at him as some kind of brute, but in that particular historical context. I have come to communism because of daddy Stalin and nobody must come and tell me that I mustn’t read Stalin. I read him when it was very bad to read him. That was another time. And because I’m not very bright, and a hard-headed person, I keep on reading him. Especially in this new period, now that it is worse to read him. Then, as well as now, I still find a Series of things that are very good.” [anarchoilluminati: no source in original]

While praising Stalin's leadership, Che was always pointing out the counter-revolutionary role of Trotsky, blaming him for “hidden motives” and “fundamental errors”. In one of his writings he was underlining: “I think that the fundamental stuff that Trotsky was based upon was erroneous and that his ulterior behaviour was wrong and his last years were even dark. The Trotskyites have not contributed anything whatsoever to the revolutionary movement; where they did most was in Peru, but they finally failed there because their methods are bad” (Comments on 'Critical Notes on Political Economy' by Che Guevara, Revolutionary Democracy Journal, 2007).

I want it to be true.