G-Man, you can put the later rear end into your 55 Chevy as you were told. The plates you refered to are to mount the stock springs to. You only have to worry about where to weld the plates and can get the measurements from the old rearend. You will need to ensure you have the same pinion angle. Having said this, the new rearend is about 2 1/2" wider than the 55 so you will have to purchase wheels with the proper backspace so you can fit them into the wheel well. I am not clear about what you are refering to when you ask if the spring perches will line up. I am assuming you are using your 55 springs and are only going to attach the rear end to them.