Eric,
As you posted,
5PRC is a 1946-model code 5 = Kansas City, MO
PR = 5400
C = 3rd month = March
Your "which March, 46 or 47" question is an interesting question, that most likely cannot be easily answered. If you have the original engine, it would be interesting to see the block and head date casting codes. This would give information about the year.
For titling/registration purposes, the "which March" question does not matter. You have a 1946-model truck.
