As Scott says the first 3 digits in the number stamped on the flat by the distributer tell the displacement on GMC inlines. If it has the original engine there are four possibilities for 1955. They are 236, 248,270,& 302. '55 was the last year for the 236 and I have never seen one. 302s were only used in big trucks. So you will find it is a 248 most likely but maybe a 270. There are only four GMC heads. One was used only on 302s it has huge ports and a D shaped chamber. One was used on the later 270 and maybe later 248. It has smaller ports and a similar chamber to the 302 head. Next is the 270 "H" head it has the same huge port size of the 302 but has a domed chamber. All other GMCs of this family used a small port head with the domed chamber. The 270 and 302 have a 4" stroke and a taller deck similar to the 292 idea.
While all of this is fun if the door tag is right it is a 270.
LINK
