Small Block & Big Block Chevy Specific - 5.3 to 5.7 conversion

I have a 2001 Chevy Suburban 5.3. I put a 99 Trans Am 5.7 LS1 block in it but used the manifold and wiring and everything else off the 5.3 and hooked it back up. I switched the PCM to the 5.7. It runs bad. Uses alot of fuel, black smoke, runs real ruff but is drivable and runs. I was told to put the 5.3 PCM back on it and it would run beens we used all the original wiring. It will let it start for bout 3 seconds then die. I was told to send the 5.3 PCM off and have it reflashed cause its been sitting for over a year, but dealership says it won't help-that the two blocks are not compatable cause of the crank and cam shaft and it will never run right. Anyone have any insite or help in anyway would be appreciated. I have added an aftermarket security and remote start to it since the conversion. Could that be why it starts then dies aafter 3 second when we put original PCM back on. It codes with the 5.7 PCM on it-P0134B, P1135B, P0135B, P1133B, Po300B and now its coding B2961. We put new 02 sensors, coil-wires fuel pump, etc..

i think u will get some response if u post up in the gen 3 forum. this is the old school section:D