AC 20-138c "is for aircraft manufacturers, avionics manufacturers, installation shops, or other applicants seeking design and/or airworthiness approval for the positioning and navigation systems described in this document".
Installtion of a 430W in an experimental aircraft is accomplished per the current installation manual and AC 43-13. You can reference the Garmin 430W STC in your entry, however, the STC does not include experimental aircraft so it's really not applicable.
The average installer is not equipped to apply for an airworthiness approval (STC) per the AC, and besides Garmin already did that all that for you with their STC.
