All of these are different internally. The only one that will work properly is the one designed for your system.
Once you start substituting parts it becomes very difficult to make the other parts work
The 552 is listed as a substitute and it works most of the time, except it does not work for the six bulb systems. Various people have designed "better" electronic flashers and lights, but the originals work just fine if you service them properly
The 323 is designed for early electronic sequencers like the 1968 Shelby. The 1969 uses a completely different set up. There you should probably use the C9WY flasher
If you have a Mustang all of this is mute. The regular flasher will work just fine.
