Hmm, until I see an actual pic of YOUR MAF and how you made those ends for a 5.7L chevy motor work I'll hold off on giving a final opinion on this, but heres the vibe I'm getting.

You're using a MAF thats designed to be in a certain diameter housing that will allow a certain ammount of air through it. Now you have replaced the housing with a larger one that allows more air in without recalibrating the sensor. IMHO the sensor is probably still sensing the original air volume and giving those readings to the ECU, but the new housing is allowing much more air in. Essentially you have a big ass air leak in your intake that the computer cant account for. Thus your lean condition.

Dallas