Didn't Zubrin's design require data from martian landers? How soon could someone know the in-situ propellant scenario was an option?
We already knew the composition of the Martian atmosphere. His plans for Mars base-building required knowledge of Martian soil chemistry and other factors that were not known at that time, but that's not needed for the propellant plant, and would be gotten in a manned mission anyways.
His own timeline estimated ten years' leadtime from policy change to the first mission. You've got habitats to develop without experience with Skylab or Mir, new launch systems to design and test (in-situ propellant earth return vehicle), an aerobrake system for ships landing on Mars, and radiation risks to mitigate.
Yeah, and I'm figuring that they'd launch Skylab and spend most of the '70's working towards the Mars mission. The "we've got these Saturns laying around anyways" isn't a serious proposal from NASA, so much as an excuse to present to Congress so that the Saturn lines don't get shut down (ie, by claiming you might need to build new parts for one of them, etc).
Getting operational heavy launch capability would be easier for a (very) early version of the concept, migrating to a Saturn V follow-on than building something from shuttle spare parts like Zubrin's Ares concept, but the design's got problems for this scenario. Some stuff's going to be slower to develop than it would be today, and I suspect there's not going to have been enough data earlier than Viking to support the initial idea to the point of it getting backing.
Saturn V's got the oomph already and they'd use it in the initial missions, and it's already optimized for operations beyond Earth orbit. As for stuff slower to develop, that's going to be Mars ground operations stuff, ie, rovers, science equipment, etc. We're simply not going to get as much stuff back in an early Mars mission as we would now. But the perfect is the enemy of the good - if the political will was there, it was a doable mission, given the correct framework.
I figure they could land a crew in 1982, given the appropriate plan and not slowing down after getting to the Moon. They could do it with chemical propulsion, although they might/probably develop NERVA for the upper stage for added mission mass.