Get the schematics and a big ass dry erase board and goto town figuring out what you can tie into. Keep in mind a FWD 3800 supercharged ECM wont know what a RWD 5-speed Manual is if thats what you intend on using. The FWD auto transaxles and RWD auto transmissions would be "4T65e" vs a "4L60e". Off the top of my head I'm not sure if the electronics are the same or not between those.
Around here I could buy a complete late model F-body car with a 3800 for less than $2000 at a repo auction (with about 100k miles). I'd hate to gut one that isn't wrecked though. I always wanted to take a 3800 f-body car and blend it with a 1956 Thunderbird replicar. Very simular wheel base dimensions between them.
BTW what year is your Mercedes 230 CE?
Wiki has some good info on Buicks v6 here