The best "useful" information I can give you if you are determined to pursue the project: Get rid of the 380 and get a solid 560 SL and start from there. The 560 is the better enginered and built of all of the 107's and is where any serious attempt at a V12 W107 should start. Take that to heart. I'd really would love to see you pull it off. (I've built a V8 pinto myself.)
I think it will require an extensive front suspension and subframe modification. The tranny tunnel is really low in that chassis and getting the 12 to sit low enough is gonna require a lot of room that isn't there to start with. The 107 doesn't have the typical "firewall" space so you have to get all your "extra" space low and forward. You're likely to have interference at the steering gearbox, idler arm, and possibly upper control arm inner mounts, as well as the subframe, drag link, and damper. The subframe ties the front suspension together and any serious modification will essentially require a redesign of the front suspension. As mentioned before, the brake booster is gonna interfere with the intake which is huge. You're gonna have to get creative there. Something like a hydraulic brake booster maybe? A rack and pinion may just solve the gearbox, idler arm drag link issues. The 560SL has a diff I believe strong enough to hold up to a 12 and hydraulic rear shocks. The hydraulics in the rear have a "ladder bar" affect in that it lifts the rear of the car during a squat, (ie hard launch)
No flame war intended. I should have realized that "trust me" wouldn't suffice for a reason "why not" If you are up to the task, then kudos, go for it. But as you may start to realize it will be a major undertaking. If you're emotionally attached to the 380 SL then build up a V8 and concentrate on upgrading the chassis to 560 specs. You'll find that to be much more rewarding. You'll be able to upgrade different things along the way and be able to enjoy the car in the meantime. The option is to strip the whole front end off the car, position the engine/tranny, and re-engineer the understeering, stoplight terror around it.