How do you say year 1978 with just 78?

In conversation i would say something like " I passed my O Level maths examination back in 78".

In print i might use an apostrophe to show that the number is abbreviated

eg. "I wrote my first program using punched cards but that was in '78.".

I don't really see what the problem is. If you are using the past tense then you are probably not refering to the year 2078.