As I said, the main difference is a .090" mill job, resulting in a smaller combustion chamber and higher compression. I don't believe there's a noticeable difference in the port size or shape, so breathing should be the same regardless of the casting number. The access hole for the temperature gauge is different on the older head, so fitting some types of aftermarket temp gauges can present a problem.
Milling the 54/55 head moves the intake valve closer to the piston, resulting in an interference problem unless the valve is recessed into the head the same distance as it's milled, and it's possible some shimming of the position of the rocker arm stands and possibly some shortening of the intake valve stems might be required to restore the proper valve system geometry. Any oldtime engine machinist would have been aware of these differences and the way to compensate for them, but it's getting necessary to hold a séance to ask most of them a question these days!
Jerry
