Lucas has to be misrembering things too. Jabba wasn’t designed as a slug until ROTJ when a bunch of different maquettes were presented to him. So the idea human Jabba was going to be a Slug stop motioned in, that can’t be correct.
If the scene was kept in 77 Declan would have been in the movie.
According to Paul Hirsch in “In a Cutting Room Far, Far Away”, Lucas came up with the stop-motion alien idea when ANH was in post-production, since Mulholland’s acting was very hammy (obviously, they couldn’t actually accomplish this.) The film’s script presented in “The Art of Star Wars” notes that Jabba was a slug-like creature (with eyestalks), but this script had some minor changes from the actual script, and this was one of them. So it seems he came up with the slug idea shortly after the scene was filmed.