No Start

I'm working on a 2000 Impala with the 3.8L engine. The engine won't start. We started diagnosing it and found a bad fuel pressure regulator (replaced it) and then notice it had coolant coming from the TBI. Knowing the 3.1L/3.4L/3.8L are notorious for bad intake manifold gaskets, we tore in and replaced them (yes, they we're cracked like usual). After we pressurized the coolant system (post gasket replacement) we found the upper intake (phenolic) was broke. Replaced it (to the tune of $298.00). That's the long story. We know it dumped/pumped a lot of coolant through the system (pulled half a coffee can of coolant from between the upper and lower intake manifolds), but we still can't get the thing to run. We checked and it doesn't have any engine error codes. We have good spark (25kV), we have gas (noid lights say injectors are firing AND we got wet plugs - gas wet, not coolant wet), the thing won't run. Replaced the ICM (Ignition control), won't start. Replaced the PCM (computer), won't even crank! Engine error code on new PCM says security code mismatch P1631. Questions: How do you reset the security system. Old (original) PCM has security light flashing. "New" PCM has security code. Any clues as to why this thing won't run? HELP!