Using a 16 bit controller is a good choice. Using an 8 bit slower controller means most of the work you'll be doing is trying to get the processing streamlined enough to work within the hardware.
It sounds like you're pretty new to this, though. Making a sequential EFI that works on a daily driver is not an easy task. It sounds simple, but in order to get the car to idle properly, cold start, hot start, throttle advance, ... it gets very complex. It's not outside the realm of the hobbyist, but making one from scratch is alot of work.