My guess (wait for some one else, before accepting my guess):

F 4 25 H
F = Flint, MI
4 = April
25 = 25th day of April
H = manual transmission - 1957 truck engine (235)

I'll bet that over 80% (>90%?) of the people who have been assured (even by family members) that it is the original engine find out that the engine is from a different year. Owners and shops thought nothing about swapping engines to keep these trucks working.

Does it have 4 slits in the valve cover? (54/55 did, but the VCs were easily replaced)

Does the water pump body go above the bottom of the front of the head (54/55 did)? (post a photo - or email me a photo, if you want)