Since you have a 4 speed have you thought about just going to a 9" rear with a lower numerically number gear to get a more comfortable cruise rpm? A (" should run you around 150-250 then a driveshaft modification is not to hard by a shop. So it costs you 500-600 bucks and lower rpms. I would do that before a t5. Also you could probably get a different gear set for your existing rear to help. Just the rear would make a big difference in your motor.

Also what fluids are you leaking?