Here I've scaled a side elevation of the ship from the book reference drawings alongside a screencap of the ship from the original film. I've scaled the drawing as close as I can to mirror the image of the ship from the photo. It isn't prefect, but it's more than enough to show that the reference drawing is definitely off. The most notable inconsistency is the length of the aft section---it's clearly far too short in the drawing. You can also see the fins are also more steeply raked on the model. Finally the shape of the forward section doesn't see quite the same as the drawings.
I certainly am curious to know what references were used to create the drawing. If he was working straight from screencaps alone it's still hard to see how he could have missed this. If by some chance he was working from archival materiel (such as original drawings) then perhaps the original design was meant to be shorter and the actual mock-up was made larger.
I would really like to see a really good large image of the ship in profile, assuming there is one somewhere amongst all the Flash Gordon
serial footage. There isn't any such image in the original film.