vapor lock?

I have a 65 chevelle w/400. Went on a couple of long runs this past summer and had some problems with vapor lock when i'm stuck in traffic. The car has a high volume mech pump. I wanted to install a feul return after the secondary inlet to the carb, but where do I put the regulater? In front of the primary inlet or after the secondary inlet? And what size return line should I use? The car is set up now with #8 line to the pump, #8 from the pump to the regulater, #6 from the regulater to the carb. While in traffic the engine does't get hot, but I get vapor lock. Any info would help Thanks.