You should not have your return right next the supply line. I converted my orginal sending until to 1/2" and used that as my return. (this is on my 56 belair). You are causing the fuel to be stirred up (for a lack of a better word) which is causing sir to get into the fuel lines. Someone can probably explain it better. But I have always been told that the return should come in the top of the tank. Hope this helps.
Tom
Tom