Thank you for the comments guys.
@ Willy, Bob and Al: I'm afraid I will have to pay to get rid of the shell. It might be a punishment for making them melt non-Swedish steel
@ Kev: workers are coming next week to open our garden-shelter and install a large garage door there, so I will soon have a place to put the Sierra first to strip it off, and the to move the roadster's chassis and all my stuffs in a suitable space (remember that for now, I'm working in the old classroom where there is just a normal door, just large enough for letting the chassis out on its side, not more!). So I will soon be going forward!
@ Mike: Wow, 25cm sounds a lot! So, do you think I can fit the big ugly EFI from either engines under an Equinox bonnet?
@ Oscar: yes, I also saw people making their own manifold, but if I can avoid this part of the work, I would be more than happy. Problem with Saturn's parts is where to get them, as they no longer exist? Did anybody take over the moulds ? When I'm installed in the "new" garage, I'll check both engines and chose the one I'll put in the Roadster. If you have any other information about fitting a DOHC, I'm still interested, same on my side, I'll keep you posted about that.