From what I understand, the basis of these new variants is the civil AC313, which in modified variants entered service within the Naval Aviation at first as a transport (Z-18), then AEW (Z-18J) and ASW (Z-18F). In parallel they were introduced in the PAP - not sure as Z-8AWJ or even AC313; I must admit, I don't know - and further developed into the Army Aviation variants. This was at first called Z-18A but later identified as the Z-18G ... and evolved into the wide cabin variant Z-8L.
Why these still are called Z-8 even if they are now even one step more away from the original Z-8 than the Z-18 (VIP) I don't know.