I ended up using the director and cast that are listed for most reviews on Ebert's site for matching the right movie. Even that required some tricks due to spelling errors or differences in how names were listed. I then flagged any matches that weren't unique or where the title wasn't similar enough for me to manually review. I think I only had to double-check about a hundred or so.

I didn't use the letterboxd api. Instead, I generated csv files for the letterboxd importer. I then did a csv export from their site I could reconcile to look for import errors.

Trivia: Ebert reviewed a few adult films which I couldn't import to letterboxd because the site officially doesn't allow those.

BTW, it's only $19/year for an account. I have my own account I pay for which follows the re2 account. That way I can easily see any of the re2 reviews for movies in my own watchlist.