[dsraven]

Check the frame and body dimensions in the assembly manual for specifics, (google 55-59 factory assembly manual, pick the tribe site for free dowload), but the 58 and 59 have a longer front end and thus longer frame. The rad support frame brackets are also further forward for the same reason but look like the same actual brackets. The hood hinges are also different and are a single contained unit with spring bolted onto the firewall in the same spot I think. The Cavs are supposed to be interchangeable for all the years if you buy an aftermarket cab but in reality there are a few little differences like the bump out for the wiper motor space. I believe the front spring hangers are located in the same place etc so other than the frame being longer ahead of the spring mounts they are the same from there back.sheet metal is all different ahead of the cab.
I'm sure you have this figured out but you wanna have the frame blocked up at your desired rake angle and levelled side to side before doing the MII cross member install. Most guys install them a little ahead of the stock axle center line because the truck will sit lower than stock and, because the front fender wheel opening is sorta round forward of the tire and angled behind the tire, it makes the tire look like it is being stuffed into the rear of the fender the lower the body drops over the frame. It's really good to mock up the ride height with the tire size you want so you can figure the new axle center line.