hey guys, iv'e got a 68 camaro with a 377 , the 355 rear thats in it now is leaking like crazy. from the pinion seal . i replaced the seal once , and it's still leaking. so as my rear end knowledge is limited to what iv'e seen done on tv, i don't whant to bang the thing any worse than it allready is , i'm ganna get this rebuilt by a pro. if i go from the 355 posi that's there now to a 390 or 410 what kind of rpm increase am i looking at. now at 60 mph i'm about 28 or 2900 . the car is a 4 spd m21. any ideas would be great.