yes you do need O2 sensors. they moniter the air fuel ratio and you car will run like crap without them. they are required for any car with fuel injection or a feedback carb. you WILL fail emissions testing if you dont have them. and considering you have a saleen supercharged motor you would be a complete moron to not use O2 sensors.
now rear O2 sensors you can do without. all they do is moniter the efficiency of the catalytic converters. they were required as part of OBD2. however depending on where you live you will fail emissions for not having rear O2 sensors. you can get MIL eliminiators which mimmick the signal that teh rear O2's give to the computer, however those will also fail a good visual inspection.