Your 1955 engine is a high-pressure 235, but the original 1953 truck engine was a low-pressure 216 (or, a low pressure 235 in largeer 1953 trucks). The truck 235 changed to a higher-pressure lubrication system in 1954 (see the link at the bottom of this post).
Since many trucks have "traded up" to the high-pressure 235s, there are after-market high-pressure gauges available from parts vendors.
The high vs low oil-pressure, in general, is due to differences in the oil lubrication system. The older, lower-pressure lubrication system, among other differences, had an open directed spray of oil to the rod bearings (sometimes mistakenly called a splash-oil lubrication). The high-pressure system has oil fed to the rod beatings through the crankshaft.
http://www.1954advance-design.com/Documents/EngineeringFeatures/slides/Page42.html