The dual sync Accel distributor has a crank signal output and a cam signal output, right? Assuming that is the case, then you've got everything you need for a full sequential system. (Note to others: You need to go to an MS3X or MS3-Pro, rather than an MS2, for the full sequential version.)
By the way, just a quick aside on full sequential versus batch fire or bank fire. I have noticed almost no difference in how engines using these different methods run and idle. There just doesn't seem to be a big difference there. One potential advantage with the full sequential is that you can tune the timing of the fuel injector squirt to optimize power output or idle charactersitics of each cylinder. But that is really, really fine tuning the engine. I tried that on my SOHC and didn't see more than a few HP difference despite varying this all over the place. On most engines I think full sequential is not really necessary.
You can use the O2 sensor in one of two ways. The way I do it is called open loop mode, and to do this you run the engine, log the O2 sensor data at different points on the VE map, and then adjust the VE map manually to try to get the A/F ratio where you want it. The second way is called feedback or closed loop mode. In the software for the EFI box there is also a Target A/F ratio table. You can fill this table in with whatever ratios you want. For example, you might fill in most of the table with 14.7, which is the idea stoichiometric A/F ratio for most driving on stockish engines, and then fill in the part of the table where you have wide open throttle with a richer number like 13.0:1. Then, if you enable feedback from the O2 sensor, the EFI box will read the O2 sensor output and the actual A/F reading, and adjust the fuel injector pulse to try to match the value in the Target A/F ratio table.
The problem with closed loop mode is that, as Ross mentioned earlier in the thread, a big cam can fool the O2 sensor into thinking you are leaner or richer than you actually are at less than full throttle. With your cam, I think running closed loop would be a mistake. You could run it just at full throttle, and that would work well, but at idle and partial throttle openings it is not going to give you the right tune. Its not that hard to tune with open loop, so I think I'd go that route if I were you.
Regarding one or two O2 sensors, I usually run 2 just to see if there are differences bank to bank on the engine. Usually there aren't. Also I'm pretty sure that the EFI box is set up to go closed loop with only one sensor, and special electronics are required to use more than one. My friend Scott Clark has an 8 O2 sensor setup, where all the sensor outputs are put on a CAN bus that communicates with the MS3, and each O2 sensor controls its associated fuel injector in closed loop mode. Basically, it individually tunes each cylinder in real time during the dyno tune or dragstrip run. Lots of wires, but the result is really cool...
