A few points:

 With a 40' tower and a 10' diam. rotor I would place the top guys at 35' to minimize the moment and deflection. Just be sure to have them below the blade tips

 A 40' tower with a thirty foot guy radius is  conservative (nothing wrong here except taking up space, the extra cost of guys is insignificant.

 A guyed pole as you descibe is a soft structure and is usually "run" with somewhat loose guys.
Because of this, frost heave is normally not a concern. Many commercial towers in this size range are erected without any or with minimal concrete at the base. The only concern is the shear force when the tower is raised, I some times use a temperary cable to the far anchor to resist this force while pulling up the tower, if it is at all questionable .16" or so square will be plenty of bearing on the soil once it is up.

 A cubic yard at each guy is more than enough and allows you to have the top of the concrete flush with the ground. An anchor that was buried 3.5' to 4' could be 1/3 of this, however the rods bringing the anchor point to the suface then need to be galvinized.