Wilf Wild 1937 said:
The last official (!) 54,000+ gate was Sunderland in the 5th Round of the FA Cup in 1973.
The capacity was then about 54,500 with seats in the North Stand.
It was trimmed shortly afterwards to 52,600 after the report into the Ibrox Disaster was published.
It stayed at that level for a number of years.
You've answered a question here that I used to wonder about occasionally, which is why we could have a crowd of 54,478 for the Sunderland game in February 1973, after the North Stand was seated, yet when I was a kid (I attended my first match in 1975) the capacity was always given as 52,600. I assumed that for some reason they must have left a little part of the stand as terracing in the first year, then put seats in that part afterwards. However, the capacity being reduced to implement requirements following the report into the Ibrox Disaster makes sense.
I think the capacity stayed at 52,600 until the summer of 1989, when all terraces had their capacity reduced (by 15% or 20%, IIRC) in the wake of the Hillsborough Disaster; as I recall, this was originally billed as a temporary move pending the Taylor Report but, unsurprisingly in the circumstances, the measure remained in force until grounds had to go all-seater. I assume that it was the first time ever that the capacity of the ground had dropped below 50K (apart from perhaps on a temporary basis when they were working on the new North Stand in the seventies).
Anyway, based on worsleyweb's figure of 54,290, it looks like we'll fairly soon have our biggest home gate since 1973.
EDIT - Though, if Chicago Blue is right, we'd beat the Sunderland game and the 55,023 that saw the game against Derby in April 1972, meaning we'd have the biggest crowd since 63,326 watched the famous 3-3 Manchester derby in November 1971. That one won't be topped even after they've developed the North Stand, though.