I agree with Nigel, and with the OP in principle. However, RT is describing a mindset the family had while the father was still alive, long before they found the box. Between his death and the inspection of the box, the view on the father was of course the same, but did not matter that much.

In other words: the narrator primarily regrets his lack of judgment during the father's lifetime, that's why he mentions "long before". Something like "I wish we then had known about the box!" or "If only long ago we'd found that box!" would convey the "long before" aspect.

If I were to "cover" the song, I would not deem the flaw grave enough to justify such an incisive change. The whole song tells a powerful story, also very idiomatic of the genre - clearly above average.

Future lyricists can learn from RT and from this thread. Poetry is a tough job.