Recently I have read about radioteletype devices capable of sending text messages via radio waves already in the first half of 20th century. Unfortunately, these teletypes used Baudot code, which is too limited for my needs. Ideally, I would like to have robust, wireless text transmission over large distances using at least 256-character encoding, including control signals (no transistors allowed, just vacuum tubes).
Initially I thought that devices should use QAM, since it allows to send multiple "bits" in one signal, but I fear 256-QAM would be too difficult for simple electronics of the era. Is it so? Would it be possible to build 256-QAM equipment in 1940s/1950s? If not, what other technology can I use?
I am specifically asking about the possibility of developing enhanced radioteletypes capable of being deployed in vehicles like trucks, ships of aircraft, not about some experimental equipment working only in lab conditions.