The SLV-3 would use a standardized configuration based on the Atlas D missile for all launches with the exception of different widths for the top of the rocket depending on the upper stage being flown.
[2]: 45–46 The SLV-3 had thicker gauge tank walls to support the weight of upper stages as well as upgraded engines and removal of unneeded ICBM hardware such as retrorockets.
Although the main engines had greater thrust, the verniers were detuned slightly in the interest of improved ISP (vacuum specific impulse).
General Dynamics Convair division was contracted to make modifications to the Atlas booster and design the cryogenic Centaur upper stage with the engines for the upper stage being furnished as Government Furnished Equipment via a separate contract between the U.S. Government and Pratt & Whitney.
[11] Atlas Centaur (SLV-3C) was designed to deliver the Surveyor moon lander missions to soft landings on the lunar surface as part of the buildup for the Apollo program.