I wouldn't necessarily assume that. If it is indeed obvious that several answers need treatment due to length, presumably it's because the letter length doesn't appear anywhere in the grid. I'm not sure it makes logical sense to go on to assume that two 13-letter answers which do appear to fit in the grid must be the others.
At the risk of teaching my grandmother to suck eggs, I approached it by ignoring the treatment aspect initially and seeing how far I could fill the grid. If you can solve the 13-letter clues, and enough others, you may be able to work out which goes where by seeing where the 4-letter clues fall.
Sorry if this is a bit elliptical, but I don't want to give too much away or ruin your challenge