Dave,

I would go with 100# of AMS applied to get your sulfur and 21 units of N.  

For the rest of your nitrogen needs, I would determine how many lbs of urea would needed to meet your intended goal.

If you can apply this mix of AMS and urea ahead of a rain event, then using untreated urea would work fine.  

If scheduling around a rain is too complex, then a stabilzer like Agrotain or Excelis Maxx (blue smurf color) added to all the urea would work fine. 

This assumes you have a supplier capable of blending fertilizer and applying stabilizer in their blender.

Bill