The wiki article is not quite correct in its description - the project envisioned using the Italian 127/54 Compatto, rather than the American Mk.45. That seems to have been an assumption of whoever wrote the wiki, as the cited source (Conway's) does not claim this, either.
I actually have not seen much mention about this in Italian sources, but I suspect this might have been seen as a cheaper alternative to procuring the second pair of
Audace-class destroyers? The first pair were procured in the late 1960s, and the Marina Militare's intent was to follow this with a second pair - which would have been named
Animoso and
Ardimentoso. Budget constraints and competing budget priorities - including the upgrade of Tartar/Terrier ships to Standard Missile - ultimately lead to this being pushed off to the late 1980s (the
Durand de la Penne-class, which initially carried these names).
As
@Dilandu said, the idea would be to replace the 5"/38 Mk.38 mount forward with a 127/54C, and the aft mount with a Mk.13 GMLS.
Beyond the financial issues - the need to procure an additional pair of Mk.13 GMLS, associated American electronic equipment, and another 120 RIM-24B/C Tartar - the
Indomito-class were simply not designed to handle the amount of top weight required by a guided missile destroyer (which had a much more extensive radar suite), and nor the same ship movement. This is well reflected by the
Impavido-class, which were developed from the
Indomito on similar hull lines. They were larger and heavier ships, and incorporated a stabilization system based on two (or three, from another source) Denny-Brown stabilization fins, which were used to reduce the amplitude of the roll oscillation and thus potential limits on operating missile systems based on ship motion.
The Mk.13 GMLS can replace the 5"/38 Mk.13 1:1 in terms of volume footprint, though it is about twice as heavy (43.5t vs ~90t). Structurally, the Mk.13 mount on the
Impavido-class is actually placed one deck level above where the Mk.38 mount on the
Indomito-class, and further forward (atop the aft deckhouse). So theoretically on the
Indomito, it would result in less topweight... but then you have to account for the illumination radars (2x SPG-51) or 3D FRESCAN (SPS-39), to which the
Indomito-class had no equivalent.
Another limit worth noting is data management. The
Impavido-class were too small to fit the Italian Navy's computerized combat system of that era - SADOC 1 - in addition to the Mk.13 system, and the
Indomito-class would have been no different. This was one of the drivers behind the substantial increase in size represented by the
Audace-class, which were able to fit SADOC 1 from the start. It was not until the development of the modular SADOC 2 that such a system could be installed on smaller vessels. The
Impavido-class were the first Italian ships to receive this system, during their MLU in the mid-1970s.
So, all in all, not a great idea and the money for this could have certainly been spent better elsewhere (as, indeed, they were).
On the subject of improbable guided missile conversions - there is actually an even more unique proposal from this period.
The two Capitani Romani-class light cruisers retained by Italy after WWII, as is well known, were converted into destroyer leaders from 1953 to 1955. From 1963 to 1965,
San Giorgio (former
Pompeo Magno) was rebuilt with a CODAG propulsion system and replaced the light cruiser
Raimondo Montecuccoli as the school ship for the Naval Academy at Livorno (though, this was only a partial career, and she was still used for regular squadron service). She would continue to serve in this role until her decommissioning on 1 February 1980.
Around 1965-66, her sister,
San Marco (former
Giulio Germanico) came under consideration for modernization as she reached the potential halfway point of her life. One proposal would have seen her modernized in a similar capacity as her sister. Another, more interesting option was to convert her into guided missile cruiser with a Mk.10 launcher for RIM-2 Terrier. Unfortunately, there are very little details on this project available, but based on the comments as to why it was not executed - the wear on both her hull and her propulsion over her career - it seems apparent that she would have retained her steam turbine propulsion plant. Which does conjure the rather amusing image of a 39-knot Terrier cruiser.
In any case,
San Marco did not have a very active career beyond this point. After a taking part in a last series of exercises in the winter and early spring of 1966, she was placed in reserve and left in availability, pending major works. These never happened, and she was ultimately decommissioned on 1 February 1970.
