the leading letter, at least by what I have read on stovebolt, is the crank/bore/rod sizes.
http://www.stovebolt.com/ubbthreads/...1832#Post11832
so the bore on a B prefix engine is +0.005" and standard crank and rod bearings. 228 is the size, you cant make a 228 block into a 270, the deck heights are different, and 1950 definitely came with a 228, so chances are excellent that you have a 228.
FC could be 1947-1950, my 1947 is an FC, with a three digit serial. see if your last 4 digits of your serial plate inside the cab match the last 4 digits on the engine block.
this is everything I have learned about the GMC engines from stovebolt, so dont crucify me if my parroting of information is incorrect.
