I have a 2004 4 w/d blazer 4 dr. 4.3 that I want to swap a 5.3 ls into. Can I still use the original cpu (red-blue) without a reflash? It will still have to pass Tx. emissions

 

You MIGHT be able to use your stock PCM, but definitely not without a reflash. Probably better to use the one that goes with the engine.

It's possible to pass California smog with that swap in California if it's done "right", so I can't imagine Texas is any worse.