Delphi CZ20008 / AC Delco (reman) 88864773 for small block
Delphi CZ20005 / AC Delco (reman) 19179576 for big block and the AC Delco is "subject to core availability"
We've discussed it here in the past and I'm a little embarrassed I don't know why the Vortec small and big blocks have different distributors, but they do. I forget if someone nailed what the difference is.
However, according to Holley, the vehicle compatibility chart shows that particular part # 55923 fitting Vortec 5.0, 5.7, and 7.4.
Richard