Your A/C guy should be able to identify where the leak is coming from and he should be able to fix it. On the other hand I would think he would have tested the system when he performed the conversion...

The "hissing" from the front vents is totally normal - what you are hearing is the expansion of the refrigerant in the evaporator. You should hear the hissing when the compressor is enganged - and it will be most noticable at low fan speeds combined with low car speeds.

Your car (especially in Florida) is a bad candidate for R134 conversion - go back to R12.

I would not embark on an evaporator replacement without verifying the evaportor is leaking.