Hello All,
David S.F. Portree's "DSFP's Spaceflight History" website contains historical articles--which often contain scale data!--about planned missions and hardware (including the Apollo-Saturn V-based manned Venus and Mars flyby missions [a December 1978 launch window would have allowed *both* planets to have been visited on the same flight!]) that never came to fruition. The current article (04 February 2019, see:
http://spaceflighthistory.blogspot.com/ )--which covers the proposed Saturn IB-Centaur interplanetary launch vehicle (a drawing of the Saturn IB-Centaur is attached below), in connection with the Voyager Mars orbiter/lander spacecraft--is no exception, and:
In the late 1960s (as mentioned in L.B. Taylor's 1968 book, "Liftoff!"), the then-planned high-energy space probe launch vehicle for the 1970s would be either the Titan IIIC (without the Transtage, with a Centaur stage used in its place; this was the Titan IIIE, which lofted the Helios, Viking, and Voyager probes), *or* the Saturn IB (also topped by a Centaur stage). Both vehicles would have virtually the same payload capability. Also:
While it was recognized that the Titan III-Centaur would be cheaper, there was also support for the Saturn IB-Centaur, partly due to the fact that it would keep the Saturn IB production lines open through the 1970s. (One plan for future [1970s] manned spaceflight involved Apollo Applications missions using refurbished recovered Apollo Command Modules, mated with new Service Modules and launched atop Saturn IB vehicles; had the Saturn IB-Centaur interplanetary launch vehicle been adopted, the continuing availability of the Saturn IB would have facilitated these manned missions.)
I hope this information will be useful.