1999 Grand Prix Issues

I bought this car used 3 yrs ago. Previous owners spun bearings in motor and replaced with 03. I had numerous issues with the cooling system and overheating since I purchased. I replaced water pump, thermostat, hoses, radiator, did flush, put in a sealant. Drove fine for a while. Recently, intake gasket leak led to me spinning bearings in motor and waterlocking. I completed motor swap. Now has 97 motor, new everything in cooling system, radiator, water pump, coolant temp sensor, air intake sensor, ambient air sensor. If I run it with a thermostat it overheats quickly, take it out and it overheats while stopped at an idle. Unplug sensor and fans come on. Before replacing coolant temp sensor fans would come on at 212 sometimes, not always, engine gets close to overheat. We have been tinkering with it for a month. Not driving it currently. A zillion codes reading, from abs sensors also all new, to engine new engine sensors and code reader shows temp of -39. I verfied by vin that all sensors and parts are indentical from my old 99 to the new 97, only thing different so far is alternator which isnt an issue since mounting brackets were changed. This car seems to be back at the exact point where I was when I originally purchased it. Could it be they never had computer reprogrammed? At my wits end, only thing I haven't changed is the computer. Could it be due to new motor not matching up with computer specs? Do I need to change computer or just have it reprogrammed to fit new motor? What is an ECU? Is ECM and PCM the same? Where is the PCM located on a 99 Grand Prix?