Actually by turning the engine/transaxle around, the engine weight doesn't move towards the rear at all. Since the drive axles are about in the center of the engine the engine weight stays in the same location. Only thing swapping is the entire transmission is now just forward of the axle centerline, a good thing which moves weight towards the center of the car and not back to the back end. And the heavy chain assembly is also swapped to the front of the engine (relative to the car) and is also closer to the center of the car. Relatively light weight pulleys end up being at the back of the car for the alternator and water pump.
Yes, using the Olds 455 engine, heavy but strong. And it is not so bad compared to a small block Chevy for weight. Also plan on some aluminum heads to shave a couple of lbs. off the engine weight.