Was working on a tuneup for my 2000 LeSabre, checked on fuel pressure while I was at it I got 50psi with the engine off which according to my Haynes manual is within spec with the car running pressure dropped to 43 psi. My book doesn't say anything about running pressure, also when I increase rpm quickly pressure rises to 50psi when it increases slowly the pressure barely moves. Any help with this would be great, thanks in advance.

Yep, your fine. When the engine is running, its pulling a vacuum on your FPR which opens it most of the way to allow the unused fuel to return back to the tank. No vacuum, valve is closed all the way and no fuel returns to the tank.