Long term archival storage

I have around 20 Tb of data that I (a) want to store for a very (50
years)long time and also have available for search and download.

The data consists of two types:

(a) the preservation masters which is the data we want to keep and is
in tiff
and bwf formats among others

(b) the viewing copies which are in derived formals such as png and
mp3.

I am coming down to an HSM type of solution which a large enough front
end cache to allow us to keep the viewing copies online at all times
but which allows the archival copies to disappear off to tape to be
cloned and duplicated etc.

Anyone else doing this? Anyone got a better idea?

-dgm