Why is my 4.3 TBI running rough and very rich?

My 1992 GMC S15 4.3 TBI Blazer just started running rough and very rich won't accelerate no power. The engine was completely re built 18K miles ago and ran great up until a couple of weeks ago. Block was bored out new pistons, rings, main bearings, oil pump, Heads reconditioned with all new valves, new cam, lifters, timing gears and double roller timing chain. New MSD distributor and new Magnecor ignition leads and spark plugs.

I have replaced the distributor rotor and cap and spark plugs after it started running rough and rich. I have replaced the fuel filter and measure 12 PSI at the fuel filter with steady pressure key first turned on and engine running. The fuel filter was replaced with new. I have tried a different MAP sensor, a different TBI, different EGR valve. I have put a scanner on and the temp sensor. O2 sensor and TPS all have normal indications with the scanner as well as the MAP sensor. I have tried a different Ignition control module also. The CAT was melted and plugged so both the CAT and muffler were just replaced with new. The base timing has been checked and it is bang on at 0 degrees (wire under dash disconnected). The wire was reconnected and after all the above. The engine still runs rich and rough. I do not want to run this vehicle much in fear that I will burn up the new CAT. I feel I am down to the ECM which is original with 250K miles on it. I will be trying a different computer tomorrow night.

Do you have any other thoughts or suggestions?

Thanks in advance for any help,
Glenn