Carb spacer

[Tater]

I'm thinking of getting a spacer for my carb but I'm not sure which one to get. The type I'm looking at is supposed to swirl the air going in to the carb which makes it more dense which is supposed to add a little horsepower and give a little better gas mileage. I know that if you get one that's too short or too tall it will only make things worse. I've talked to a couple folks who have them installed and they both said, once they got the right size, it improved their mileage by as much as 2 mpg. I have a 360 with a 4bbl carb and auto trans and I'm looking for any info from real world experience. Thanks for any help.