Your engine code is certainly a puzzle.
The date code on your 55' block seems to decode as March 14, 1958. Not a problem if you believe that GM periodically cast older style parts at later years. See this discussion about the same numbered block (3733949) with a date code of J129 (Oct 12,1959)ending with David Hayward, automotive historian, supporting that idea.
http://www.chevytalk.org/fusionbb/printpost.php?tid/260623/Back to your original question. First, the format of the engine code as engine assembly plant, month, day, series/designation, (S0405XH),didn't appear until 1957.
Next, the first letter "S" should stand for "Saginaw" or "Saginaw Service".
Several Nova/Camaro/Chevelle sites stated this but I could find
no other online references to this as an engine assembly plant. (Years this plant was operational?)
http://www.nastyz28.com/sbchevy/sblock.phpNext, the 04 and 05 are obviously month and day.
Finally the suffix XH. When limited to 235 truck engines, the suffix X appeared in only the 54-56 years I believe. In years 54 and 55 there was only X, XA, and XB noting regular production 3100, 3600, and 3800 models. In 56 we go from X to XGC but never quite to XH. So, rather inconclusive.
So we have a 55' block, cast in Mar 58' with the engine assembled later than that at a service engine assembly plant?
I hope someone else can add to the engine replacement process during this time.
