IMHO, the spinnaker halyard should be run up to a turning block, not a fixed sheave in the mast. It should come out of an exit slot about six-inches to a foot below the turning block, so that it leads fair to the block. The reason for this is simple. A spinnaker, regardless of whether it is an asym or symmetrical, requires a greater range of motion for the halyard than does a genoa, since it isn't attached to a stay or foil.
It needs to be above the headstay attachment point, and if you're on a masthead rigged boat, this may require installing a masthead crane to give it some additional height or allow it to be mounted forward of the headstay.
Flying an asym inside the foretriangle is a really bad idea IMHO. First, you'll have far more chafe on the halyard, though probably less on the sheets...but the sheets are far easier to replace.

Second, it limits the height of the spinnaker.