Single, good idea! 38 to 40mm should work well. If you do a search "carberator upgrades for Vangaurds" is a pretty good read. I went twin 30's on Wilson, but the throttle cables were not easy and I still think I'm losing fuel at top end. I've been toying with a High output fuel pump or maybe 2, but it runs so nice and fast now, what more do I need. Good luck with your build