Definitely more than two options here.
- run return line back to new tank
- keep return line where it is and Tee into the line feeding the carburetor so that both new and old tank feed the carburetor. You will need to be careful of how high you mount the new tank to avoid putting excess head pressure on the old tank. Or use check/shutoff valves to isolate the tanks from each other.
-Feeding the line through the gas cap will be very difficult to seal and will be prone to leaking I wouldn't suggest that route.
- From what I understand the fuel return line is only used as a way to make the priming bulb work. While the engine is running the return line isn't actively moving fuel. Different engines might operate differently here.
If it were me I would run both lines back to the new tank to keep things simple. That or just Tee into the main fuel line and use a check valve to prevent the Aux tank from flooding the original tank.
2017 25 taps on buckets got me hooked 1 gallon of sweet
2018 51 taps on 3/16 tubing/ DIY oil tank evaporator 8.5gallons finished
2019 60 taps 7 gallons finished ended season short
2020 New 2x4 divided pan ready to get away from the headache that is steam table pans
2021 off year due to pandemic and projects
2022 back at it