I was confronted with the same issue when I installed a Garmin GTX 335 in my Beaver.
Since I did not have an integrated GPS WAAS capable system installed (read Garmin), I needed the GPX-335/345 with the internal GPS as part of my ADS-B system.
I chose to go with the 335 to meet 2020 standards, only Out is required. But you need the out to trigger the In. I will find a different solution for the in signal receiver and display.
According to Garmin the GTX 345 will provide (using Garmin Connext and bluetooth technology), "Wireless connectivity to portable device using Garmin Pilot, ForeFlight or compatible Garmin portable".
Not sure about the iFly 740b. Pretty sure Garmin wants you using Garmin!!
So now I have a new 740b. Will be deciding on a hardware solution to provide ADS-B In data that will work with it.