UPDATES:

Thanks to Jonno, I discovered that the HD-DVD video I have has 12 missing frames... (XX/)

Jonno managed to capture those frames for me; then I reinstated them in the correct place, color graded them as the rest of the movie, and I had to resync all the soundtracks... (XX/)

Good news: I finished in record time, and all is well!

Jonno sent me the DD+ (extracted to lossless 48KHz/24bit); at this quality, the uncompressed PCM is about 5.5GB! As the main english soundtrack will be PCM taken from the Signature Collection LD (next will be the PCM from NTSC Letterbox LD), a good third option could be a "simple" DTS 48KHz/16bit, retaining a very good quality in limited space (about 1.2GB).

I found also the italian, french, german and spanish soundtrack from BD: the italian is with "superstitious", while the DVD has not, so I'll use both; the french is 5.1 while the DVD is only 2.0, but I have the PCM 2.0 so I'll discard the DVD one; german was missing, so I'll use it; spanish on BD is better quality than DVD, so I'll discard the latter. I think I'll convert them to medium-bitrate AC3, to give more space to video and because the original recording is not that high quality - opinions about this?