You can NOT use that line for fuel return (or shouldn't) for a couple of reasons
First, originally, that line IS the tank vent. It went up front as you know, tied in with the evap/ carbon can system, and vented the tank. THERE IS NO OTHER vent
Look up a 69 service manual (free) from MyMopar and you will quickly see how the "un" evap tanks vented----using a simple tube welded into the upper filler neck
(While you are at it get a 73 manual from there too. Read the fuel and emissions sections to find out how this all worked)
Second, that line is likely WAY too small if you are talking about a return regulator/ EFI/ etc. If you are simply talking about a fuel/ vapor return from something like a 3rd port fuel filter with the anti-vapor lock orifice, then it will be ok, BUT YOU STILL would need to provide a vent
The fuel caps in none of these cars are vented--and that fact helps keep fuel off the fenders
If you don't provide another vent, that fitting needs to remain "up front" and "up high" to prevent siphon.