Seriously, the guy is such an impact player that you could leave his weight alone and he'd probably still be a factor at end in the 3-4. Likewise he could probably handle the OLB role at his current weight as well. Suffice to say that Peppers, if you've got him on your team, is an asset and a good one.
I believe though that the very best way to use him is in the 4-3. It's a question of best fit. He'd be great in either of the two spots he could play in the 3-4 but he's phenomenal where he is in the Panthers defense.