The issue with this is E85 changes all the time, so you'd have to constantly measure the fuel, then pop in and change the stoich and all of your AFR targets.
If it were an LSx ECU, life would be so much simpler because you change stoich and you're done. The rest of the fueling is handled in equivalency ratios (inverse lambda), so the ECU recalculates the appropriate AFR every time regardless of what the stoich is.
If you had a flex fuel ECU, it'd be easier yet again. Dave Steck makes a nice ethanol sensor for GM ECUs that's nearly plug and play. Set the PCM up and it automatically handles everything from fueling to ignition changes.
