one of our projects is a 61 vette with a gm ls3 crate engine. has a fast harness with a gm ecm. it is loaded with a stock 08 vette tune and runs extremely rich and backfires through the exhaust above 3000 rpm?? the only difference between the 08 vette and our setup is a little better flowing exhaust due to no cats and maybe a little better breathing intake setup. we have tried everything. we are new to the tuning game and are learning but are both gm certified on fuel injection back in the obd1 days. it has been a while. what do we need to check? also we are not getting any codes. the car runs almost perfectly with the maf unplugged but wont rev past 4000 unplugged. this is a chassis that has not hit the street yet. does the ecm need to learn??