Just went through this nightmare on my 70 Challenger. Found out that 70's have a nipple on the top of the filler tube inside the trunk which gets vented through the trunk floor into the right frame rail. Drilling a hole in a vented gas cap will vent the tank but also allow gas to spill out. I know!!
What you have is a filler tube from a 72-74 ebody which has a totally different venting system.