I believe the film was intended to be set in 1978. We can work this out using a few data points given within the original film.
Firstly, according to Jor-El (or his AI simulation, or whatever it was), Clark was 18 years old when he left Smallville and first entered the Fortress of Solitude.
JOR-EL: My son. You do not remember me. I am Jor-El. I am your father. By now you will have reached your eighteenth year, as it is measured on Earth. By that reckoning, I will have been dead for many thousands of your years.
Secondly, according to Jor-El again, Clark spent 12 years studying under him, before arriving in Metropolis and becoming Superman. This would make Clark 30 years old at that point.
JOR-EL: By the time we return to the confines of your galaxy, twelve of your years will have passed.
Finally, according to Luthor, Superman stated in his interview with Lois that Krypton exploded in 1948, and that he arrived on Earth three years later.
LUTHOR: In the interview he says that the planet Krypton exploded in 1948. Ridiculous little freak took three years in a rocket ship to get to Earth.
Clark (or Kal-El, as he was named at the time) was a newborn infant when Krypton exploded, so if he was 30 years old when he became Superman, that would mean the majority of the film was set in 1978.
Jor-El did appear to contradict himself though, when he stated that he would've been dead for many thousands of years by the time Kal-El/Clark reached his eighteenth year. We know Clark was born shortly before Jor-El died, so that's a contradiction right there. And Clark told Lois that Krypton exploded in 1948, and that the journey from Krypton to Earth took three years, so that's another contradiction.
We could speculate that Krypton did explode many millennia ago, and that baby Kal-El's aging was drastically slowed during his journey to Earth, as a result of his rocket ship travelling at near-lightspeed. But that wouldn't explain how Jor-El knew about Einstein and early Chinese writings.
JOR-EL: ...which Einstein called his Theory of Relativity. Embedded in the crystals before you, is the total accumulation of all literature and scientific fact from dozens of other worlds, spanning the 28 known galaxies...
JOR-EL: Early Chinese writings point out the complex relationship between...
Jor-El mentioned that baby Kal passed through six galaxies on his way to Earth, which suggests that he travelled a vast distance which would've taken at least tens of thousands of years to cover at near-lightspeed (assuming that galaxies are similarly as far apart in that universe as they are in the real world).
JOR-EL: Each of the six galaxies which you will pass through contain their own individual law...
If Jor-El died at least tens of thousands of years ago, he shouldn't have known anything about Einstein or early Chinese writings, unless he had the ability to see the future. And it seems unlikely that Kryptonians had that ability, since most of them clearly didn't foresee the destruction of their own planet.
Another (arguable) contradiction to the aforementioned timeline occurs in Superman III, when the banners/signs at Clark's high school reunion indicate that the reunion was being held for the Class of 1965. However, it is possible that Clark graduated at 17 years of age, rather than 18.