The differences between the Excelsior and the Enterprise-B aren't as significant as the differences between the eleven footer and the AMT kit. The E-B is an Excelsior with some added pieces (obviously, since it's the same model, only modified), but just about every
structural aspect of the AMT (saucer, secondary hull, nacelles) differs in proportion and size, if you use the bridge module as a common component, (and even that's taking a liberty or two because of the differences in the shape of the bridge dome). In fact, taking that route, the Constellation comes in around twenty feet shorter than the Enterprise.
We're not talking the difference between the USS Nimitz and the USS Ronald Reagan (a Nimitz class carrier with some major differences), we're talking the difference between a Nimitz class carrier and a Kitty Hawk class carrier.