The best solution for a Mars mission and colony back in the 1970's (perhaps even late 1960's) was the Project Orion nuclear pulse propulsion system.
Here's a link to good details about Project Orion:
https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)
And here's a link to a BBC documentary that went into great detail about Project Orion:
According to prominent experts involved with the Project Orion, the project was within the technological capabilities of the late 1960's USA.
The "Super Orion" spaceship version designed by General Atomics was capable of lifting a ship 400 meters (1312 feet) in diameter, weighing 8 million tons, from the surface of the Earth all the way to a landing on Mars. The spacecraft could be the size of a small town, with a similar population and complex infrastructure.
The Project Orion spaceships are still the best solution to launch colony ships from Earth and establish settlements around the Solar System.