Many / Most knock sensors have built-in filters that filter out the frequencies that are NOT associated with knock on a given engine's bore. (The frequency that knock makes varies by engine bore.) -You need a sensor designed for your rough engine bore because of this. (A 350 / 383, an SBC 400, and a BBC should all have different knock sensors.)
If you search the Holley EFI forums, you'll find some recommended knock sensors for an SBC with a 4.000" - 4.030" bore with links from Danny Cabral.
I had to rebuild my PC and I can't find some of my OneNotes, including the one that lists the specific knock sensor that I used in my 350 SBC... I'm using Holley EFI, though and it can work with almost any knock sensor.
The Holley ECUs require you to "tune" the knock sensor to get good results; I'm not sure how this works with a GM ECM...
With the modern Holley ECUs: you put crappy gas in the tank and then lug the engine up a steep hill in high gear to get some definite knocks to show up in the logs. -Then you look at the intensity of those knock events in the log and you set a % Knock Threshold in the knock configuration. (When you look at the knock log as a chart, you'll see the intensity of the TRUE knock event(s) that you generated by lugging it up a hill, then you can look back in the log for other events that created sounds at the knock frequencies but at lower noise levels (piston slap after engine startup when the engine is cold, lifter noise, etc..). -Your goal is to set the knock threshold HIGHER than those other non-knock events, but LOWER than the ACTUAL knock events with the Threshold settings. Then you can setup how many degrees of timing you want pulled when knock exceeds that threshold and how quickly you want to put timing back in after it's been pulled.
Adam