Budget EFI Kit

I have a 318/904 with Mopar electronic, Edelbrock 600 cfm on a performer intake also comp XE268 cam. I am running Doug's headers so my question is would I need two O2 sensors and could they be put just behind collectors? I am thinking seriously about this kit to improve drivability and increase milage. How much increase in mpg are you seeing out of the kits that have been installed? Joe