Decoding chart below taken from the GM Heritage Site
1955 Chevrolet Truck 2nd Series PDF.
The "2" in front of the "55" designates it as a 1955 2nd Series engine. The date casting code, located above/behind the starter on the side of the block, will tell the Month (alpha character), day of the month (1 or 2 numerals), followed by the year (one numeral) it was cast.
The "X" confirms it was originally installed in a 1/2 ton (31 Short bed or 32 Long bed) truck. No powerglides used in those trucks, so a standard pilot bushing is what's needed.
