Well, in 1957 it was the hardest climb at the cliffs. I wouldn't be surprised if it started out as a 5.9, but by the time Art Gran's guidebook came out seven years later it was already down to 5.8, right there with Jacob's Ladder...
Yellow Belly has a significant place in Shawangunk history. Its ascent by Jim McCarthy 52 years ago represented the end of the depressed Eastern standards that were a byproduct of the Appalachian Mountain Club's conservative hegemony over the Gunks climbing scene. Until that point, 5.7, with the occasional Wiessner 5.8, was the top of the Shawangunk scale, but Yellow Belly surpassed not only all the old 5.7's but was certainly also harder than Wiessner 5.8's like Minniebelle and was probably the hardest climb in the Northeast at the time.
Not only did Yellow Belly represent an increase in standards that had been two grades behind the West Coast, it also represented a psychological breakthrough for McCarthy, who went on in short order to other 5.8's, then 5.9's, and then 5.10's.