The MS-IVB-1 third stage would have had the same size and shape as the unmodified S-IVB stage, but it would have been strengthened because of the larger payload capacity and flight stresses. The J-2 LOX pump would have been modified.
The MS-IVB-1 third stage, designed for a larger payload capacity and greater stress from flight, would have maintained the same dimensions as the original S-IVB stage but with enhanced structural integrity. Modifications to the J-2 LOX pump were also planned.