Peter 005
I think you are on the wrong tram.
C129a is acceptable for PBN IF you have another navigation source. C129a is NOT acceptable for ADS-B which requires C146a.
Australia is the only country in the world mandating ADS-B for ALL IFR flights at all levels in all airspace.
The KT74 is one of about 3 available ADSB-ES transponders (Garmin, King & Trig). But it requires a C146a GPS input to be compliant. But, I don't think they have started shipping them yet. I think its still a couple of months off.
So, your KLN94 has to go if you want to fly IFR. It will need to be replaced by a Garmin GNS430, GTN650, GTN750, Avidyne IFD 440/540 or King KSN770. All of these will require a panel relay, new wiring and new antennae for you.