On my ST-60 (1980) the original (Gotoh?) block is already steel, so no need for replacement.
On my ST-80 (also 1980) the block and top-plate is one complete die-cast unit made of Zamak, so my luthier friend Simon Pinder is installing a Callaham block and top-plate.
(Zamak, often called "pot metal" or "white metal", is an alloy of zinc containing aluminium, magnesium & copper. It is quite soft & was originally used - since the 1920's - for making die-cast toys, like "Dinky" & "Corgi Toys" etc. , it's now often used as a substitute for lead (!!!) in wheel balancing weights ...& is not optimal for musical instrument parts!)
Upgrading to the Callaham has not been a simple drop-in re-placement : -
The six screws that hold the top plate to the ST-80 guitar body were almost 1mm further apart on the Callaham plate (& the current Gotoh, incidentally), so Simon has had to plug, re-drill & re-tap the holes. The string spacing is also very slightly wider, around 0.5 mm.
We've been able to use the original TOKAI-stamped saddles with the Callaham block & top-plate, so outwardly there'll be no change in overall appearance.
I should hopefully be getting the guitar back this weekend or early next week (it also needed a re-fret, re-wire & re-finish), and I'll post my thoughts after a gig or two with it.