How best to describe this...

I had an idea to do a kind of writing project where I 'translate' Francois Sagan's 1954 French novel, Bonjour Tristesse. It's a very short book (100 pages), and I thought I could do a page at a time, putting it up on a blog or something. However, I can't read French, so instead I'd be 'translating' the translation by Irene Ash, and just channeling a copy of the French. I don't know how different my version could be. Possibly very, although I'm not really aiming to modernise it or anything.

The issue is, when I idly thought of doing this, I'd presumed the book was out of copyright, being a Penguin Classic and all, but it's actually not even 60. And Sagan only died in 2008 (does that make a difference - will the book be out of copyright 75 years after that, or whatever it is?) In the UK, the only editions available are by Penguin, suggesting they own the copyright (or at least the copyright to the translation, which is presumably a separate thing?).

So I guess the issue is, if I posted a piecemeal (non)re-translated version of Bonjour Tristesse online, would I be in violation of copyright, and if so, how different would my version have to be to get around it? Publishers do parodies of each other all the time (FIfty Sheds of Grey for example), so where do the boundaries lie?