Perhaps to be on the conservative side, then: Strike length, 8 cells (since the minimum at the moment is 8 cells each).
Here's you link:
https://rhk111smilitaryandarmspage....-conversion-for-oliver-hazard-perry-frigates/
Again (and I am being polite), there is no simple answer to that question, it is still a how long is a piece of string question.
Looking at the link you provided, whoever wrote that article came up with 'guesstimates' of cost of installation to the RAN's FFG's, and that is all they are, guesstimates.
A Government could go to LM and request pricing on the '8 cells' you mentioned, is it 8 cells only? Is it part of a larger order of 188 cells? What sort of spares and support package is being included or not?
As to fitting to an 'existing' ship, again way too many variables.
You could have a ship that when it was designed and built had 'space and weight' allowances included, in that case, it would be a far simpler exercise, cut the hole in the deck and drop the cells into the space that had been provided.
On the other hand a ship that did not have those allowances during design and construction, could require a 'significant' re-work of the spaces below the deck, that would become a very expensive exercise.
And of course there are the ships sensors, does the ship have existing sensors that the VLS system can be integrated into, or does the ship need new or upgraded sensors?
Mate, sorry, but you are not going to get the answer you are looking for.