As 3B said tubless tires (22.5") came out in 1956. The new larger size tires that were available rubbed the inner cowl area so in 1957 Chevrolet moved the front axle 2.5" further forward. This move also required the wheel opening to be increases from about 39" to 42" measured across the bottom. So there is a difference between 55/56 and 57 series 4 & 6 fenders. The later year parts books only offered the 57 fender as replacement on the 55/56 trucks because it didn't really hurt to run the larger opening when everything else was the same.
I beleive the 58 frame is longer from the axle forward...since you own both examples you can confirm that for us.
The 58 hood hinges are also different so if you use the 58 cab and 55 hood you will need to modify the hinge bolt area at the cab (I believe I'm correct in saying this, but I've never done it myself).
Just move the entire axle to the frame you want to use...might make things easier and should bolt right in.
8.25 x 20 tires are the exact diameter as 9.00 x 22.5's...9.00 x 20 are the same as 10.00 x 22.5's.
Mike B
