Getting a rear end from a 4.5 is a good idea, as the 3.27 ratio will do better than the SL one.
The weakest part is the sliding joint, and the sliding sleeve inside it. All sliding sleeves are the same, make sure to get one which is not cracked. Try to get a sliding joint out o a 6.3 (some 4.5 also have it) as they are stronger.
Apart from the sliding joint and the ratio, all parts are the same, so they are at leat able to take 250hp and 51 kg of torque like the 6.3. I guess torque is more a problem that power.
What kind of V8 are you fitting? If an MB 3.5, 3.8, 4.2 or 4.5, your rear end will do fine, even without the reinforced 6.3 sliding sleeve. But you may need it with a 5.0 or 5.6.