I'm going to just rant here a bit if it's alright with everyone.
Been a while since I looked at the 340 research, just out of curiosity, has anyone checked what the answer(s) cannot be? solving the negative so to speak, simply based on language rules? Assuming it's English, and there's no extra step (probably even if there is) there are symbol-to-plain combinations that simply are impossible.
If you take any 4 consecutive symbols, you can be pretty sure they're not all the same, excluding things like "From now on I will ki(LL LL)amas, not people", or "I will take over the world with ma(SS SS)-armies". but, say... 5? consecutive letters, even across word boundaries? Ridiculously improbable. Similarly.. long vowel/consonant combos are pretty uncommon. At the very least, you might be able to put the symbols into "cannot be the same" groups. The symbol order doesn't matter, 12345 =LLLLL is impossible even if it's 54321, 14325, 23415 or even 12234, as long as they appear together. And if there's enough data, you might be able turn that into a "must be the same, because it can't be anything else" list.
You don't really need to know what each symbol is, if you know what it isn't. Add to that the fact that some symbols are so common that they exclude a bunch of letters. There are 24 of the + symbols, well, that's not a J, or a K.. or a Z.. Realistically, it's E, T, A, O, I, N, S, R, or H.. the next most common letter is D and at 340 letters, it would be expected to show up only 15 times; H at expected 20 might have an extra 4 occurrences, but I wouldn't bet on D having 9 over the expected.
And the same goes for symbol combinations. + and B together make up 36 of the 340 spots. They could BOTH be E, or T.. but the next most common letter A, is only expected to appear 28 times. Any solution where + and B have to be the same, and that letter is not E or T is likely incorrect.
It should narrow things down. It might even prove conclusively that there has to be an extra step if there are no answers that are within the realm of possible. In other words, don't look for words, look for things that can't be words.
Personally, I'm guessing there is, and that it's something like "every other letter". There are 4 double symbols, right? He knew that's how he got nailed the last time, it's unlikely he made the same mistake twice, those are likely results of the shuffling method used (...well, they could be across word boundaries, but I doubt it), every other letter might easily result in an inadvertent duplication of a symbol for a common letter.
... wow, formatting got messed up... hope it works now.