Revell 1/144 scale Saturn V dimensions?

[Michael Morris]

I'm investigating building a diorama of a Saturn S-IVb stage being loaded into a Boeing Super Guppy aircraft in 1/144 scale.  Revell used to make a nice 1/144 scale Super Guppy and I was planning on using an Airfix S-IVb stage in the model.  However, it looks as though Airfix S-IVb stage won't fit inside the internal diameter of the Revell Super Guppy kit. ☹️
Can anyone tell me what the external diameter is of the ribbed section of a Revell 1/144 scale S-IVb stage?

Thanks