I wonder why you need a fuel return line with a carburetor?
At first, I thought you were running Electronic Fuel Injection that needs a return line because the system is closed and without a return line, any air would have to go out the injectors. IFI is under 55-psi, so running it anywhere is ok.
Then, I got to the word, 'carburetor'. Carburetor bowls are open to atmosphere, so any air in the line burps out in the bowl as a natural thing. Even with a regulator, there is no need or purpose for a return line. Fuel pressure is only at a few PSI with relation to atmosphere pressure.
Mechanical fuel pumps mounted on the engine must not overcome restrictions in the suction side, so a low fuel line before the pump works well. After the fuel pump, you can run the fuel line anywhere you like. They normally don't go to the firewall because there isn't much distance between the pump and carburetor. Suspend your fuel line away from metal and you will be ok. - Dave