They could do it back then too.

If you dig past the internet comments and read some of the "primary source" books, the picture of the leaks is very different. I can't remember the exact book I read it in, but the author states there was a tank sealant, and it lasted around 50 hours (I think), before it needed to be replaced.

This is somewhat backed up by the Jenkins book [1] which talks about the time consuming process of replacing sealant, and the Graham book [0] that is the source for the Wikipedia claims on expansion. It talks of different sealants used, and how leaks were precisely noted and collected in _shallow_ drip trays.

[0] https://books.google.co.uk/books?id=dX5cCgAAQBAJ&pg=PA129&dq...

[1] https://books.google.co.uk/books?id=lEpWE748QUsC&q=sr71+seal...