gas filling solution?

[78 Big10 454]

When I bought my square new in 1978, there was no mandatory smog testing in Kommiefornia. I dumped the cat and went with straight dual exhaust and swapped out the filler neck with a model from a leaded fuel truck. That did the trick in preventing back wash every time I filled up with gas. I swap in the original non-leaded filler once every two years when I get it smogged.