Wouldn't the single 4-barrel 360 horse 413 that was standard in the 300 letter car have been considered HP? It had a hotter cam than the 340 horse 413 that came in the New Yorker.
Could be, I've just never had the chance to scrutinize an original one in person to see how it's stamped. The smaller stamping characters do look different (they look to be the same as all other regular production engines) unlike the earlier ram induction letter car engines (which had larger font due to being built on the Marine line).
Here's a pic of my 63 300J engine, notice the font difference.
