I am returning my Duster to some very limited street duty - slicks, wheelie bars, alcohol, (fuel not cocktails ) etc. I'm having a custom aluminum 22 gallon fuel cell built due to the horrendous fuel mileage expected.

As to venting the cell, I've considered two -6 vents, one -6 vent, or one -8 vent. Anybody have any experience with this?

The rollover valves are fairly expensive once you include the fittings and hoses. I don't mind spending for what is needed, but I hate wasting money....

The amount of fuel leaving the cell is what you need to consider. I have imploded a plastic cell or two without the proper venting. If you have an AN8 leaving, then an AN8 vent should be used. If you have two AN8's leaving, then upsizing it, will be need to keep from creating a vacuum. My A1000 did a number on a plastic fuel cell of mine.

I run a -8 with a roll over bulkhead fitting.

http://www.summitracing.com/parts/sum-g3114/overview/

Some of them can be plumbed in line however they seem to be more expensive.

The filler cap is also vented so a single -8 is plenty

uh-oh, I hadn't thought about the cap. So it will be venting alcohol fumes into my trunk... where my battery sits? That's going to create new problems.

The cap on my cell is a non-vented aircraft style deal. It has a safety blow off but is not normally vented.

Pro-Werks makes screw on replacement caps that are not vented. Use that and vent sizes equal to what you have feeding the pump and you will be good.