The vapor return line setup came from the factory on 400s and AC cars. I can only speculate why but I suspect it had something to do with the need for extra fuel delivery. The single line system on the 350 cars works fine as long as that's what it's used on. The only heat issues with this setup might come from an owner modification, such as adding headers or if the fuel line has been moved closer to any exhaust components. As long as everything is in it's stock configuration you should be fine. No worries.