Computer
 

Is there a way to wire ECM to run v6 in my rat rod ?

I'm certain it can be done, but PCM will need re-programmed. This is because you rat rod will not have a lot of the systems that the PCM runs and is used to seeing on the data bus. I would discuss your exact needs with an experienced PCM tuner. Do you also need PCM to control the transmission?

Kinda off topic, but since we're talking about pcm reprogramming,

Can a pcm be programmed to delete certain systems, like the cat, egr, evap, things like this that a fellow might not want on an offroad toy, but doesn't want to see a cel all the time?

Yes it can

Yes, to both questions

A tuner can remove the VATs (vehicle anti-theft) from any of the Blazer, Silverado, Van, etc, computers that run the 4.3, and then it's just a matter of throwing in a harness and plugging it all in. Removing things like EGR, EVAP, cat converter monitoring rear O2 sensors, etc, would make the retrofit that much easier.

It's the same situation as swapping a V8 into one of these Blazers that originally had a 4.3, just minus 2 cylinders in the end computer programming :)