My impression is that with continued high levels of funding (probably higher than the projections), and a tolerance for risk, it would have been quite possible.
The life support systems were not there in 1970, but they would have had a whole decade to develop, test, and iterate. The Mars Program was not a standalone thing - the IPP called for a LEO space station plus propellant depot, HEO and lunar orbiting stations, and other infrastructure, as well as STS and a lunar tug, all of which would have required long-duration life support systems, radiation protection outside Earth's magnetosphere, and long-duration microgravity exposure. On a side note, thousands of tonnes of water is ridiculous; you'd have a water recycling system set up. There were lots of ideas for that being worked on through the decades.
Plans called for something along the lines of a 50-man space base in LEO and a 12-man space base in lunar orbit - that's significantly more man-hours of microgravity exposure over a decade than the ISS has given us over two to three decades.
Would it have been uneconomical, astronomically expensive, and possibly pointless (depending on philosophy) in terms of blood and treasure spent (and I mean blood, a few deaths along the way are probable)? Yeah.
Would there have been a catastrophe some point along the program resulting in a three or six year delay? Probably.
But I suspect it would have gotten to Mars in the end.