As an amateur framebulider, I'd say that unless you're willing to spend about 4-500 dollars, your frame is done. That bike needs a new top tube and upper head lug at a minimum. I don't know much about repair (having only done one headtube replacement and one rear triangle rebuild) - *I* would also replace the headtube, because that would make my job easier.

Seeing as how the bike is from the 70's - from an unknown maker - I would start shopping for a new one.

I would agree with BT's assessment of poor mitering being the death of that lug. In my limited experience, I have not seen a frame start to fail in the middle of the lug. Keep in mind though - 531, 753, 69000 - whatever - that frame is 30 years old - nothing is unbreakable.

If you're strapped, you could *TRY* and do a ghetto braze - but the longevity of a repair like that is dubious. I would strongly advise against it in such a critical area.